Field values can be added to these statements using the _fid_X=YOUR_VALUE syntax. In fact, you are already doing this when you enter the Record ID for the parent record here:
&_fid_8=" & URLEncode ([Record ID#])
So in your case to add another field you would just another statement like this for your other field:
&_fid_11=" & URLEncode ([Action])
Which would end up looking like this:
URLRoot() &
"db/" & [_DBID_WORKFLOW] &
"?a=API_GenAddRecordForm" &
"&_fid_8=" & URLEncode ([Record ID#]) &
"&_fid_11=" & URLEncode ([Action]) &
"&z=" & Rurl()
As a sidenote, you may have noticed I split this formula onto multiple lines. This is perfectly fine, and I would suggest doing so when you have longer formulas like this so you can see exactly what's going on.
