Adding wsa headers to SOAP call

Question

When calling a SOAP WS we have to specify wsa in the SOAP header. It will look like this:

            <env:Header xmlns:wsa="http://www.w3.org/2005/08/addressing">
                           <wsa:To env:mustUnderstand="true">_enpoint_</wsa:To>
                           <wsa:Action>_action_</wsa:Action>
            </env:Header>

We have auto-generated the proxy from the wsdl using wsdl.exe

To avoid having to modify the auto-generated code, I wanted to add a new partial class doing this job. I like this approach to do it, so I ended up with this code:

public partial class WebserviceProxy
{
    private const string DEFAULT_WSA_NAMESPACE = "wsa";

    private string _wsaNamespace;

    public string WsaNamespace
    {
        get => _wsaNamespace ?? (_wsaNamespace = DEFAULT_WSA_NAMESPACE);
        set => _wsaNamespace = value;
    }

    public string To { get; set; }

    public string Action { get; set; }

    protected override WebRequest GetWebRequest(Uri uri)
    {
        WebRequest request = base.GetWebRequest(uri);

        request.Headers.Add($"{WsaNamespace}:{nameof(To)}", To);
        request.Headers.Add($"{WsaNamespace}:{nameof(Action)}", Action);

        return request;
    }
}

Issue is that this code is not working and I get an exception, when calling base.GetWebRequest(uri):

System.ArgumentException: Specified value has invalid HTTP Header characters.
Parameter name: name
   at System.Net.WebHeaderCollection.CheckBadChars(String name, Boolean isHeaderValue)
   at System.Net.WebHeaderCollection.Add(String name, String value)
   at IcamWebserviceProxy.GetWebRequest(Uri uri)

As I'm not sure that I'm on the right track, I'm seeking help here.

Even if the mentioned issue is solved, I still have to specify the correct wsa namespace as Heder attribute.

[edit] URI (tested against a SoapUI endpoint running on the same machine):

-       uri {http://localhost:9000/mockDataExchangeAdminservice}    System.Uri
        AbsolutePath    "/mockDataExchangeAdminservice" string
        AbsoluteUri "http://localhost:9000/mockDataExchangeAdminservice"    string
        AllowIdn    false   bool
        Authority   "localhost:9000"    string
        DnsSafeHost "localhost" string
        Fragment    ""  string
        HasAuthority    true    bool
        Host    "localhost" string
        HostNameType    Dns System.UriHostNameType
        HostType    DnsHostType System.Uri.Flags
        IdnHost "localhost" string
        IsAbsoluteUri   true    bool
        IsDefaultPort   false   bool
        IsDosPath   false   bool
        IsFile  false   bool
        IsImplicitFile  false   bool
        IsLoopback  true    bool
        IsNotAbsoluteUri    false   bool
        IsUnc   false   bool
        IsUncOrDosPath  false   bool
        IsUncPath   false   bool
        LocalPath   "/mockDataExchangeAdminservice" string
        OriginalString  "http://localhost:9000/mockDataExchangeAdminservice"    string
        OriginalStringSwitched  false   bool
        PathAndQuery    "/mockDataExchangeAdminservice" string
        Port    9000    int
        PrivateAbsolutePath "/mockDataExchangeAdminservice" string
        Query   ""  string
        Scheme  "http"  string
        SecuredPathIndex    0   ushort
+       Segments    {string[2]} string[]
+       Syntax  {System.UriParser.BuiltInUriParser} System.UriParser {System.UriParser.BuiltInUriParser}
        UserDrivenParsing   false   bool
        UserEscaped false   bool
        UserInfo    ""  string
        m_DnsSafeHost   null    string
        m_Flags DnsHostType | AuthorityFound | LoopbackHost | NotDefaultPort | MinimalUriInfoSet | AllUriInfoSet | HostUnicodeNormalized | RestUnicodeNormalized    System.Uri.Flags
+       m_Info  {System.Uri.UriInfo}    System.Uri.UriInfo
        m_String    "http://localhost:9000/mockDataExchangeAdminservice"    string
+       m_Syntax    {System.UriParser.BuiltInUriParser} System.UriParser {System.UriParser.BuiltInUriParser}
        m_iriParsing    true    bool
        m_originalUnicodeString null    string
+       Static members  

[edit2] The exception is being thrown from HttpWebClientProtocol.GetWebRequest when returning the webRequest (last line):

protected override WebRequest GetWebRequest(Uri uri)
{
  WebRequest webRequest = base.GetWebRequest(uri);
  HttpWebRequest httpWebRequest = webRequest as HttpWebRequest;
  if (httpWebRequest != null)
  {
    httpWebRequest.UserAgent = this.UserAgent;
    httpWebRequest.AllowAutoRedirect = this.allowAutoRedirect;
    httpWebRequest.AutomaticDecompression = this.enableDecompression ? DecompressionMethods.GZip : DecompressionMethods.None;
    httpWebRequest.AllowWriteStreamBuffering = true;
    httpWebRequest.SendChunked = false;
    if (this.unsafeAuthenticatedConnectionSharing != httpWebRequest.UnsafeAuthenticatedConnectionSharing)
      httpWebRequest.UnsafeAuthenticatedConnectionSharing = this.unsafeAuthenticatedConnectionSharing;
    if (this.proxy != null)
      httpWebRequest.Proxy = this.proxy;
    if (this.clientCertificates != null && this.clientCertificates.Count > 0)
      httpWebRequest.ClientCertificates.AddRange(this.clientCertificates);
    httpWebRequest.CookieContainer = this.cookieJar;
  }
  return webRequest;
}