Auto populate one module field from another

Auto populate one module field from another

I have one module called Campaign Leads and another called Facilities. The Leads module has a lookup field where I select the facility associated with the lead. When I select the facility that I want to associate with the lead I want to auto-fill a field in the Lead module called Facility Email that already exists in the Facility Module. My goal is to be able to send a mail merge to the facility email after associating the facility with the lead. I want to send a mail merge with the lead information.