Open manage | Database | fields
Find DropDownUSers and double click it to bring up field options.
Click the storage tab and specify global storage.
This makes any value entered in this field accessible when the script enters find mode and you set up your find request.
Don't know if you've set this up to select a USer by ID number or name. The process is the same, but the field you specify for entering search criteria is different. I'll assume a name for now, but you should be able to use an ID number instead with one small change to this script:
Enter Find Mode  //clear the pause check box
Set Field [USers::UserName ; Users::DropDownUsers ]
Set Error capture [on]
Perform Find 
Thanks. That did the trick. I was afraid I would end up in Find Mode but found myself in Browse.
I have to say it to took me fifteen minutes to figure out the Set Field GUI. it is not at all intuitive that "Calculated Result" should be the source field with no operators. Results are normally targets rather than sources. The Help on Set Field does not addresss the GUI. It doesn't even say what the "Specify target field" checkbox is supposed to do.
Anyway, I'm back on track for my "total immersion" FileMaker Pro language learning.