Add create post to group homepage

I want to use New Custom Content in group homepage panel to put a simple form for posting content into that group.
I put html code of form and it works well, but once I logout and login again it doesn't function because of validation error of form token.
What kind of code (php code?) I can use to create this form. I also found drupal_get_token() function inside drupal_prepare_form() which is responsible for generating token, but I don't know how to use it inside my custom panel.

Thanks in advance