WordPress itself does not provide any forms in the frontend apart from comment forms. You need a plugin for the form functionality in any case. Almost every form plugin allows to change the processing of submitted forms via hook. So you can also address an external API after the successful submission. As I said, this works with pretty much every form plugin, so pick one: https://ww.wp.xz.cn/plugins/tags/forms/
Thanks @threadi !
can you give me some further advice on what you mean by “via hook”?
I tried using WPforms and a form block from Astra. Both force entering an email to receive responses and I did not see a functionality to change the submit button logic.
Hooks are provided by WordPress and many plugins so that other developers can put their own implementation in these places. An introduction to this can be found in the developer manual:
https://developer.ww.wp.xz.cn/plugins/hooks/
WPForms provides many hooks which are mentioned in their documentation: https://wpforms.com/developers/
With this hook you can e.g. add your own implementation after successful submission of a form: https://wpforms.com/developers/wpforms_process_complete/
I assumed from your question regarding an external API that you are a developer and would program such an addition yourself. If this is not the case and you need support, you can certainly find someone here who can help you: https://jobs.wordpress.net/
