URL Button Return URL

I have a button on the [NEW CASE] form that gives users the opportunity to create a new record [NEW KEY TERM] within another table which then populates the option list for [KEY TERM] on the [CASE] record. 

I have a button that works BUT i can't seem to get the RURL right - I want to return to the existing form/current record within the Cases Table

Cases Table = bpw9r2twc
Key Terms Table = bp2kau43a

URLRoot() & "db/bp2kau43a" & "?a=API_GenAddRecordForm&dfid=2&bg=1" & "&z=" & Rurl()

Please help!