You can simply use HTTP-requests to validate a field. A HTTP request is sended to an API or webservice, and the result decides if the field will validate or not.
If you need to populate some fields like lists, datagrids, checkboxes or radiobuttons with your own data from, for example, your database or a webservice, Peggy Forms also provides easy to setup features for this.
If you need the submissions posted to your API or webservice, Peggy Pay provides a webhook for you. Also when you use this feature, you can use values from the API/webservice HTTP response in your thankyou page.