I don't think you can add a scroll bar directly to the screen. However, you could try this as a work around:
Unlock last DataCard on the form.
Expand the Height of the datacard to force the form below the screen and present a scroll bar.
Add buttons/controls to the DataCard. Make sure the card is selected when adding the control.
Not sure if I would implement the above idea, it's kinda ugly and might cause some problems, but if you have to scroll and there is no other way then you could try it out.
By the sounds of it what you need is a "Fluid Grid" control. Unfortunately, there is no way to add these in a straight-forward way to existing screens - but there is a work around.
It's not the most straightforward method but it has reliably worked for me before.
Of course, the best way to get a scrollable screen is to build it that way initially using the New Screen > Scrollable Screen option. This might not always be an option however such as if you are using the tool to automatically generate an app from a datasource.
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