Setting default options for Multiselect Listbox on iPaaS form

I have a form that has a multiselect Listbox. I want the form to be able default to different combinations of options based on different fields on the form. How do I get it to do that?  To make it simple I've got example choices for a,b,c, and d (see "Example Choices" screenshot). So far I've discovered that if setting the value of the listbox to a||1 via some trigger like a button it will select that option. I can't seem to figure out what would let me have it select A and D.

I've tried setting the value to these different permutations so far and no luck:

• a||1,b||2
• [a||1,b||2]
• a,b||1,2
• [a,b]||[1,2]
• {a||1,b||2}

Any ideas?