URL Button to "pick-up" a task (change to current user)

I saw a similar post for this, but couldn't really find a resolution in there that works for me.  Here's the situation.  In our Project Management app we have tasks that remain unassigned.  I want to create a URL button that when clicked changes the "Assigned to" filed to the current user, and also changes the "status" field to "In-Progress."  The latter part of this works fine, but the former does not.  I cannot seem to get the "Assigned to" field to change, it just remains blank.

This is the thread to which I was referring.

This is the current button formula code: (token redacted)

I have also attempted to use "&_fid_125=URLEncode(User())" &

//This utilizes the API to edit the [Assigned to] (fid_125) field. It changes the [Status] (fid_12) to "In-Progress".
URLRoot() & "db/" & Dbid() & "?a=API_EditRecord" &
"&rid=" & [Task ID] &
"&apptoken=xxxxxxxxxxxxxxxxxxxxxxx" &
"&_fid_125=ToText(User())" &
"&_fid_12=In-Progress" &
"&rdr=" &
URLEncode(URLRoot() & "db/" & Dbid() &
"?a=dr&rid=" & [Task ID])