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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