I took this from the Magic Buttons App.
It might give you the right idea.
URLRoot() & "db/" & [_DBID_CLIENT] & "?a=API_EditRecord&apptoken=d9eascudwjxs2sesxkjccwgtq9r&_fid_28=Completed&_fid_153=Completed&_fid_143=100&rid=" & [Record ID#] & "&rdr=" & URLEncode(URLRoot() & "db/" & [_DBID_PROJECT_SNAPSHOTS] & "?a=API_AddRecord&apptoken=d9eascudwjxs2sesxkjccwgtq9r&_fid_6=" & [Project Name] & "&_fid_7=" & [Status] & "&_fid_8=" & [Priority] & "&_fid_9=" & [Est Start Date] & "&_fid_10=" & [Est End Date] & "&rdr=" &
URLEncode(URLRoot() & "db/" & Dbid() & "?a=dr&rid=" & [Record ID#]))
// Edit the record and change the status to Completed then redirect to
// Adding a Project Snapshot record to the Project Snapshot table then redirect to
// displaying the original record
