Default value for the comment field

In a certain transition screen I send an email as post function using JEMH notification.

This works, but our customers complain that they cannot follow the thread, and it would be helpful to append the previous comment (incoming mail) to keep up with the topic.

So I would like to have the latest comment pasted in the comment field when I start the transition, similar to what happens in an email client when you click the reply-to function. Of course this has to be done BEFORE the post functions are called.

Can anyone please point me in the right direction? I have been digging with no conclusive result or path so a solution.

Thanks in advance!