Adding child records that rely on lookups in new forms

I was able to do it like this:

Create a code page (copy and paste code below), change page 2 to whatever page or tab you need:

<script>
const params = new URLSearchParams(window.location.search);

const parentRid = params.get("parentrid");
const parentDbid = params.get("parentdbid");

window.location.href =
  "/db/" + parentDbid +
  "?a=er&rid=" + parentRid +
  "&page=2";
</script>

And then change your formula to this:

URLRoot() & "db/" & [_DBID_TABLE_ALIAS]
& "?a=API_GenAddRecordForm"
& "&_fid_6=" & [Record ID#]
& "&nexturl="
& URLEncode(
    URLRoot() & "db/" & Dbid()
    & "?a=dbpage&pageID=72"  //Change 72 to the ID of the code page you created
    & "&parentrid=" & [Record ID#]
    & "&parentdbid=" & Dbid()
)