Use Google Analytics to track visitors from a specific website

I already have GTM set up and tracking properly. I want to track from a new source (website) that is not in the list of filters: First User Source>Dimensions and see if they click on a booking link so that I know if they are worth the advertising money. I think I have everything I need, I just need to understand how to add/configure to get this information.

This is on a Joomla site, which shouldn't make any difference except that there may be a Joomla extension that accomplishes this.