Autofill a Lookup

Autofill a Lookup

Hi there, 

I've got a report with my customer portfolio. When I swipe left (or right), it runs a workflow:

openUrl("#Form:New_Event?Customer=" + Customer_Name,"popup window");

This workflow opens the Form New_Event. Also, I would like that autofill the field "Customer" that is a Look Up from the form "Add_Customer"
"Customer" field is a dropdown, and it's not a string, so the code above doesn't work...

Any idea?