as found in here, however, these method are obsolote, so if you are using unity 2017.2 and below, use Send(), if you are using 2017.3 and on, use SendWebRequest().
I’ve the same problem (with “using UnityEngine.Networking;”, obviously):
error CS1061: Type UnityEngine.Networking.UnityWebRequest' does not contain a definition for SendWebRequest’ and no extension method SendWebRequest' of type UnityEngine.Networking.UnityWebRequest’ could be found. Are you missing an assembly reference?