How to write better this part of code in method

Question

I write this code

public Task<TResponse>  CustomGetMethod<TResponse>(HelperUrl helperUrl,List<KeyValuePair<string,string>> query_string,string ParametersForUrl = "")
        where TResponse : class
    {
        string queryString = string.Empty;
        if (string.IsNullOrEmpty(ParametersForUrl))
            queryString += ParametersForUrl;
        if (query_string != null && query_string.Count > 0)
            queryString += $"?{String.Join("&", query_string.ConvertAll(a => $"{a.Key}={HttpUtility.UrlEncode(a.Value)}"))}";

do you have idea for get better ?