SetFocus is not supported in the VBS of Flexible, but I have a workaround for your application if you are interested.
Example, Create (3) screens (Screen_1, Screen_2, Screen_3). Create (3) String tags (Barcode_ID, Barcode_Workorder, Barcode_ProductID). Set start up screen to Screen_1. On Screen_1 place one IO Field on the screen and tie it to Barcode_ID. On Screen_2 place one IO Field on the screen and tie it to Barcode_Wordorder. On Screen_3 place one IO Field on the screen and tie it to Barcode_ProductID.
Now open your tags and locate Barcode_ID, in the properties click on Events, under Change Value set a function ActivateScreen = Screen_2. Do the same steps for Barcode_Workorder (ActivateScreen = Screen_3), and Barcode_ProductID (ActivateScreen = what ever screen you want)
When placed into runtime or downloaded to a panel you will have screen_1 displayed with the focus already on the IO Field for Barcode_ID string. When you scan it will place your string into Barcode_ID and the enter will activate the change value event of the tag which will change the screen to screen_2. Now the focus will be on the IO Field for Barcode_Wordorder and the same will occur again when you scan the next barcode.
This work around will give you the ability to scan (3) barcodes back to back and store them in 3 string tags. I don't like using 3 screens but this does work.