So you would make a URL formula field like this example
URLRoot() & "db/" & [_DBID_xxxxxxxxx] & "?a=API_GenAddRecordForm"
& "&_fid_YYY=" & totext([Record ID#])
& "&_fid_WWW=" & totext([some other numeric field])
& "&_fid_ZZZ=" & urlencode([some other text field])
& "&z=" & Rurl()
If you look at the URL formula for any system generated ADD record button, you will see a similar format.
The first line gives the start of the URL. You will need to get the DBID value from the advanced properties tab for the table. for example it might look like [_DBID_ORDERS]
The YYY needs to be the field ID# on the child table for the field called [Related Parent] (whatever your parent table is called. That is how the child gets connected opt the parent.
The WWW and ZZZ are just the field ID numbers of the field that you are populating. They are on the usage tab of the field or else you can choose to show them on your field list.
For numeric fields you need to flip them to be text
For text fields, you need to wrap them in that URLENcode, as URLs do not like spaces or special characters.
The very last part is optional. If upon save you want to return to the parent record after you save, then leave it in. If you want to "stick the landing" like in gymnastics, then leave it out and upon save you will stick on the child record.
