What should I write in the post action of this front end post form?

You need to make your processing form a template and apply it to a page if you want wordpress to recognize the form. Then you can set your action to the name of the page with the template applied to it and viola!

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)