Query Custom Form Fields using v2 API

My company has created a form field for submitting new tickets. This form field contains a number of custom fields. I would like to query one of these fields and return the set of tickets that match the query/filter.

I can query the API for some custom fields e.g.

https://sisense.freshservice.com/api/v2/tickets/filter?query="approval_status:'Awaiting%20Security%20Approval'"

this works.

I can see the field I want to query for an arbitrary ticket:

https://sisense.freshservice.com/api/v2/tickets/29048/requested_items

But the returned fields from the form don’t appear to be queriable.

What is the approach for querying or filtering via the form fields?