@retool_team Thanks so much for responding! I got that working now. I didn't get that the "Default Values" have to be the same format as the "Values". Which is absolutely reasonable and I didn't know why I didn't come up with this earlier.
I'm still struggling with updating the cell value on the table. I have tried (and failed) to run a JS code on "Save" button click on the modal:
table1.selectedRow.data.sizes = modifiedSizes.values
modifiedSizes being the multi select component on the modal.
Could you please help me out with this one as well?