I'm using Fluentx.Mvc from Nuget for this. 
Install Fluentx.Mvc from nuget
You need to include in your code: 
using Fluentx.Mvc;
and the code to call a external URL with post:
First create a Dictionary like:
Dictionary<string, object> 
          objData = new Dictionary<string, object>();
and insert values:
objData.Add("name", "John");
objData.Add("city", "NY");
After that, use return from Fluentx:
return this.RedirectAndPost("http://yourexternalurl", objData);
In your external url, you get values :
string strName = Request["nome"];
string strCity = Request["city"];