Posting from a multi form control

I’m trying for the first time to post multiple comma separated values to a field using a for comtrol.

I chose multiple from my form control
I used the multiple in checkbox in my post variable and it either posts just one value or none at all depending on the different things I tried.

I even added a paragraph field above my form control to output the “value” and it shows the correct comma separated text
Why won’t it post 837,2010 as displayed?