What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
I was searching, but I was unable to find documentation from Whatsapp about how to embed a call button.
I can see there's some HTML code that will help you to display a phone number so someone may call, but our Form Builder does not seem to allow customized HTML codes.
Maybe you could provide us with more details or the code that you're using to embed the button, you should also try getting the form's source code and add your custom code there.
This guide will help you to get the source code of your form: How-to-get-the-Full-Source-Code-of-your-Form