ソースコード

public static void SetBasicAuth
( 
    this UnityWebRequest request, 
    string               username, 
    string               password 
)
{
    var s             = $"{username}:{password}";
    var encoding      = Encoding.GetEncoding( "ISO-8859-1" );
    var inArray       = encoding.GetBytes( s );
    var basicAuth     = Convert.ToBase64String( inArray );
    var authorization = "Basic " + basicAuth;

    request.SetRequestHeader( "AUTHORIZATION", authorization );
}