<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.4/jquery.min.js "></script>
var theValue = GetUrlKeyValue("ID",false,unescape(GetUrlKeyValue("Source")));
document.getElementById("Project_x0020_ID_83d3d8e5-1352-4fba-bd8b-86bddaf97847_$NumberField").value = theValue;
Solved! Go to Solution.
I could not get the formula to work to grab from the &Source part of the URL so I went with the path of least resistance and instead added the variable I needed to the URL using instructions as provided here: https://community.nintex.com/t5/Nintex-for-SharePoint/Add-variable-to-URL-on-Nintex-Form/td-p/64186
and then in turn I was able to use the GetQueryString function