Excel: how to create an interactive popup form

Hi,
first of all thank you in advance for your help.
I have this problem:
I have a list of sites ("Sites").
In a different sheet ("Agents") I have a list of agents. Each agent is available for some of the sites.
I would like to have a way (maybe a button for each site) to open a popup in which I have a list of only the agents available for that site, so that I can select just one of them. Selecting an agent would put his data in the site line.

I attached an example