Is there anything like an "after update" command to put the selected choice into another field?

[Ray]

Each record has one site and one user which are selected from different lists.  Is there anyway to populate a single field with a single response .

Site 1

  U1

  U2

  U3

Site2

  U1

  U2

Destination Field

So if the user selects Site 1 U3 I want U3 to appear in the DESTINATION field.

I know I can do this with @Calctext but I would have to add every site user into an IF function and that seems pretty cumberson.

I\In MS Access this is simple with an After Update Set Value MAcro or vb code.

Any help appreciated.