Is your List of companies a value list such as that found in a drop down or pop up? Or is it some other kind of list like a list view or portal where you click a button?
You may not need to use a variable at all for this, BTW.
I tried it as a drop down and then as a drop down in its own window.
In which case, you don't need a variable though you could indeed make that work.
If you define your field with the value list as a global field, you could set up the onObjectSave (drop down) or OnObjectModify ( Pop up) script trigger to perform a find script such as the examples in this thread: Scripted Find Examples
If your value list formatted field is setup as match field to a related table, you may be able to use Go To Related Records or even list the matching records in a portal beneath the field.
Or you can use the script step
Set Variable [$YourVarirable ; Value: YourTable::YourValueLIstFIeld ]
Sounds like I am going at this all wrong and that I have a lot to learn. I think I will hit some of the training manuals and then come back if I still need help. Thanks so much for getting me going in the right direction.