I want to have a download button for our software that takes the user to a form.  However, I want to make sure that the user has first logged in so I can identify who the user is when they see the form.

I imagine I can do something like this by setting up a form page that is only visible to authenticated users.  When the user follows a link to that page from elsewhere on the site, if that user isn't logged in, then they get the login page, which is what I want.

However, they may not know what to do or why they got that page.  I'd like to be able to explain to them that in order to get to the download, we'd like them to log in or register.

The best solution would be for us to be able to add some page-specific explanatory text as to why they are seeing the login form.  That way, I can explain "Oh, you meant to go to the downloads?  Please do this first."  That shows that they're definitely in the right place.

As a work-around, it might be sufficient to customize the login page in general and explain that to get to our resources like the forums and downloads, they need to login or register, but that's not as desirable as a specific message that shows the user they are still in the process they started.

How might I approach this?

Ted