Copying a value from one module to another automatically

Copying a value from one module to another automatically

Scenario: Invoice module has an "Implementation Engineer" value with a list of names. When one is selected, a number of tasks and emails go out. What I'd like is to also take that value and apply it into the Account module associated with the invoice, which will then fire off a number of actions I have configured there.

I'm pretty sure I can do this with a custom function, but no idea where to begin.