Pass multi-select field values from stateless form to Page variable which feeds criteria for a gauge on the same Page

Pass multi-select field values from stateless form to Page variable which feeds criteria for a gauge on the same Page

I have a stateless form w/ a multi-select checkbox and a Gauge side-by-side in a Creator Page.
I've set up my page variable as the criteria for my gauge (selected records->field contains pageVariable).
But I can't figure out how to assign the input values of the multi-select to the page variable.
In the page builder, when configuring the stateless form and selecting the field values tab, the only option is to "Configure field values to be prefilled on loading the form" where I can assign the page variable. I want the operation to go the other way, FROM field values TO the page variable. Do I do this in the page script, and what is that syntax?
Thanks!