How to set a people field in a list, e.g. “convert” AccountName to Lookup ID?

A call to `api/web/ensureuser` using the SPHttpClient is needed. It ensures the requested user is loaded into the SharePoint site userinfo. Using the given `AccountName` like `i:0#.f|membership|bob@somewhere.org.nz` from the search as parameter. The returned SPUser contains the `Id` field for the Lookup column.

I implemented the following method:

  public EnsureUser(userName: string): Promise<ISPUser> {
    console.log("SharePointDataProvider.EnsureUser( \"" + userName + "\" )");
    var data = {logonName: userName};

    return this._webPartContext.spHttpClient.post(
      this._ensureUserUrl, 
      SPHttpClient.configurations.v1,
      { body: JSON.stringify(data) } ).then(
        (value: SPHttpClientResponse) => {
          console.log("SharePointDataProvider.EnsureUser FullFill: statusText:\"" + value.statusText + "\"" );
          return value.json();
        },
        (error: any) => console.log("SharePointDataProvider.EnsureUser Rejected: " + error )
      ).then((json: ISPUser) => {
          console.log("SharePointDataProvider.EnsureUser FullFill: Id:" + json.Id +" LoginName:\"" + json.LoginName + "\"" );
        return json;
      });
  }

My mini ISPUser interface:

interface ISPUser
{
    Email: string;
    Id: number;
    LoginName: string;
    Title: string;
}