Populating $subject and $message with Post info in mailform template

[heavi]

I am trying to create a button that will bring up the ?send message? form so a user can send an email to another member in response to a post. In addition the email will be populated with the post?s title in the subject and the post?s message in the textarea.

I know that the send message form uses the ?mailform? template from sendmessage.php?do=mailmember function and that the mailform template uses the variables $subject in the title and $message in the text area.
I know I need to populate $subject and $message with the info from the post

My thought is to use the link in the postbit template

sendmessage.php?$session[sessionurl]do=mailmember&u=$post[userid]&p=$post[postid]"

adding on the ?p=post[postid]? to the end of the link

My question is whether or not I am on the right track and if so, how do I get the information from p=post[postid] into $subject and $message in the mailform template?

Thanks