If you want to have a form on a page, there are plugins that allow that; a popular one is “Contact Form 7”. But if you want a specific ‘action’ page to fire when the submit button is used, then it might be best to create a page template.
The template could include the form code and the ‘action’ of the form tag could call another page (and template) that would process the form then display a page.
Learn more about templates here: https://codex.wordpress.org/Templates .
And it would be best to put the templates in a Child Theme (if you aren’t using your own custom theme), so any theme updates don’t overwrite your changes. (Although custom-written templates aren’t overwritten on the theme update, unless you have changed a theme-supplied template.)