There are a couple of ways you could employ to update a custom process attribute in AgilePoint. If you have the premier license, it's available as a stencil under Premier Extension. This is pretty handy when you have simple, straightforward values.
However, when you need to manipulate data or perform complex data calculations prior to the update, you can make use of the Managed C# code stencil instead. It can be found under Advanced Extensions.
With just a simple line, you're on your way:
api.SetCustomAttr(pi.WorkObjectID, "YourAttributeName", "TheValue");
Retrieving?
string x = (string)api.GetCustomAttr(pi.WorkObjectID, "YourAttributeName");
