Select default value for bound combo box?

I'm not much of a MS Access guy, so I may not word this correctly...  Here's the rough situation:

I have a three tables (offices, users, trainings) that look something like this:

Offices:

officeid           officename
1                    Seattle
2                    New York
3                    Tokyo

Users:

userid            username           currentofficeid
1                    Bill                      1
2                    Jim                      1
3                    Peter                  2
4                    Hiro                     3

Trainings:

trainingid       userid            trainedofficeid
1                    1                    1
1                    2                    1
2                    1                    3
2                    2                    1
2                    3                    2


Etc...

And I have a form that has a combobox that selects the user, and another one that controls the trainings.trainedofficeid value (displays values from the offices table and inserts the selected value into the trainings table).  That works great.

What I want to do, though, is have the second combobox default to whatever the users current home office is.  But if you select a different office, the new value should be inserted into the appropriate record in the trainedofficeid field.

Not sure if that makes sense...  The second combobox (the one that selects an office) should be bound to the trainings table in that any change you make to what's selected in that combobox will update the value in the traingings.trainedofficeid field.  But...  I want the default value selected in that combobox to be whatever the user's current office is.

And I can't figure out how to do that.

Thanks.