SOLVED.
There are some subtile differences based on the deployment method...
FOR the pure URL method
the HTML tag used <a href="https://[app].caspio.com/dp/[appKey]?CustodialAccountNumber=[@field:Institution_accounts_CustodialAccountNumber]">Details Page</a>
Subforms (tabular reports) followed a similar construction when constructing the src element of the iFrame.
If you are using hosted html (NOT the Caspio HTML pages - I could not get those to accept any parameters), then I recommend the following video: https://www.youtube.com/watch?v=1uPz1cOwNiU
