The code you posted here
& "&_fid_100=" & URLEncode ("5")
is the correct code if in fact the field for {Related Team] is fid 100.
What I suggest to debug this is to reveal any error message like this
var text AddCEmailServices=
URLRoot() & "db/" & [_DBID_Estimate_Detail]
& "?act=API_AddRecord&_fid_41=" & URLEncode ([Record ID#])
& "&_fid_100=" & URLEncode ("5")
& "&_fid_40=" & "TBD"
& "&_fid_89=" & Now()
& "&disprec=1";
$AddCEmailServicesURL
and then comment out that javascript code.
That will just run the URL formula and you will see if that errors out on something.
When you do that javascript refresh, it hides any error message.
