Perform extra actions on post save

Dear WordPress community,

I got stuck with a little problem. Maybe it’s just a little hint that will lead me to the right track.

I want to add an extra form to the editing screen of a post, but not a form that contains custom post data to be saved. I ned to perform extra actions with the content from the form when the post is saved. The problem for me is that I can’t grab the $_POST content from my extra form and that I don’t know at which point I can include my custom actions when the post is being saved.

I tried it with ‘add_meta_boxes’ action and all the related stuff, but at the moment I don’t have a clue how I can perform other actions than saving default or custom post data using this method – or if I have to do it in a completely different way.

I hope that I could point out what my problem is in an understandable way. Thanks in advance for any helpful hint!

Kind regards
winnewoerp