How to pass an Encoded ("application/x-www-form-urlencoded") object with array within a query string in HttpClient?

Question

I am trying to encode ("application/x-www-form-urlencoded") an object in Angular using HttpParameterCodec. This object has a field as object which in turn has array of string. object structure as mentioned below.

export class SearchRequest {id: string = undefined;partnerIds: PartnerIds = undefined;}


export class PartnerIds { partnerId: string[] = undefined;}

Now when I am trying encode this object with utility code (which works fine with plain object) mentioned below, It is not in correct format due to list of string in it.

generateQueryParams(searchRequest) {
    let httpParams: HttpParams = new HttpParams({encoder: new CustomEncoderComponent()});
    Object.keys(searchRequest).forEach(key => {
      httpParams = httpParams.append(key, searchRequest[key]);
    });

    return httpParams;
}

I tried with below query params format but it did not work.

1.   id=123&partnerIds=XYZ&partnerIds=ABC
2.   id=123&partnerId[]=XYZ&partnerId[]=ABC
3.   id=123&partnerId=XYZ&partnerId=ABC

Please suggest How to pass this (SearchRequest) object within a query string in HttpClient?