Dynamic dropdown list based on values from another module

Dynamic dropdown list based on values from another module

Hi, 
I can't find how to prepopulate a dropdown list with existing values from another module. 

I have a field on Potentials named "Salle" and it's a lookup field fetching information from Products module. 
The 3 products fiels are: 
  1. Assis (number field) 
  2. Debout (number field) 
  3. Cabaret (number field) 
Those 3 fields appears in my Potentials separately, but I would need their "values" merged into a dropdown list for my users to select only one.  

Any help is highly appreciated! 
Thank you

Kind regards