How to assign values from lookup field depending on picklist value?

How to assign values from lookup field depending on picklist value?

Hello, 

I am looking for a way to do this… 

In the Invoices module there is a lookup field with the name "Textmodule_Introtext" that links to a custom module with the name "Textmodules". 

And there is also a picklist called "Subscription_Type" in the Invoices module, and I want to automatically assign the value "Textmodule_Introtext" depending on the value from "Subscription_Type".

I.e
  1. Value A from "Subscription_Type" should load/be assigned to ---> value A from "Textmodule_Introtext"
  2. Value B from "Subscription_Type" should load/be assigned to ---> value B from "Textmodule_Introtext"
  3. Value C from "Subscription_Type" should load/be assigned to ---> value C from "Textmodule_Introtext"
  4. etc.
How can I do this? 

Thanks a lot for your help, 
M.