While it's possible to give your line items portal an object name, use go to object and go to portal row in your script to enter the data directly in the last, bottom row of the portal, I prefer this script:
Set variable [$Job ; Value::inventory 2::JobNumb]
Set variable [$Item ; Value::inventory 2::ItemNumb]
Go To Layout [LineItems]
Set Field [LineItems::JobNumb ; $Job]
Set Field [LineItems::ItemNumb ; $Item]
Go To Layout [original layout]
Subsitute your names for mine as needed.
I did have the first method you described working last night, but felt like it was a little gaff when I was using it. I did try the script you suggested, and it ended up working out great. I also inserted a custom dialogue that asks the user:
"How many $equipname's would you like to add?
There are X available for the selected dates
Desired QTY: _____"
Thank you again for the help!