Hi I am desperate for some help. 

I have a custom Module (Onboarding) that creates a record in another custom module called (Amazon). When the new record is created the look up on (Amazon) is completed based on the current (onboarding record). 

I need to link the records the other way, so a field update workflow can update the (Amazon)  record when required. Can someone help me with the function please. Essentially on create of (Amazon) pre populate the Look up field on onboarding with its ID.