To clarify, you want to add a new control so the user can enter a value that will be the max value for Qty Ord. Is this for validation purposes for use in later processing or for this particular task?

It's for validation purposes, but I've found that my requirements are more complicated than I expected. Instead of changing a field, I need the button to open a new Frame where I can input the value and have a drop down list for the Unit Of Measurement.

I assume you are using Customizer, which means you are limited in what you can do using vbScript. One limitation is you can't create new window forms. I would suggest getting the requirements for the customization solidified first. Once done, you can determine if it is something that can be done using Customizer, or if you will need to have a modification done to the task by a Sage 500 Developer.