Titanium.Web.Proxy how to modify request Body

Question

any here, i want use Titanium.Web.Proxy capture http/https request Body, the request is from selenium, i want started Chrome the first page request.Method is "POST", in fiddlerCore maybe use "oSession.utilSetRequestBody(Body)" be can sure,but in Titanium.Web.Proxy, i try to be use "await e.SetRequestBodyString(Body)" I got with a Error. seem that can not be tamper with the Get request as a Post request.

public async Task OnRequest(object sender, SessionEventArgs e) {
        if 
    (e.WebSession.Request.RequestUri.AbsoluteUri.Contains("baidu.com"))
        {

            e.WebSession.Request.Method = "POST";
            var ss = "sdsdsd=ffdfd";
            await e.SetRequestBodyString(ss);

        }


    }

E.g: try to visit https://www.baidu.com/, the Method is GET, i want modify be POST, so that,i use "e.WebSession.Request.Method = "POST";", but when executing to the "await e.SetRequestBodyString(ss)", it aways the Error.