Hi JW, So would the custom html page just have a button on it? if so, you can just add the button directly on the form, and create the URL to create a new record, using a formula text field:
<a class='Vibrant Success' target='_blank' href=' " & URLRoot() & "db/" & [_DBID_TABLENAME] & "?a=API_GenAddRecordForm" '</a>
You can add more fields to the url, so that those fields are already populated from the previous page, by adding the related table field id# (_fid_#) and the current table field name that correlates. For example:
<a class='Vibrant Success' target='_blank' href=' " & URLRoot() & "db/" & [_DBID_TABLENAME] & "?a=API_GenAddRecordForm&_fid_7=" & URLEncode(Today())" '</a>
However, if the HTML page has more than just a button, then create a javascript page and attach it to the header of the HTML page. Grab the button element, and then just trigger the button using an API call, like API_AddRecord, or API_GenAddRecordForm. Just pull in the Record ID# of the Parent table and the table ID, and the API call will do the rest. Look at the API documentation to find sample xml requests to help you along.