Changing the options in a menu from one to multiple

I am missing something very obvious here. I am simply trying to change the menu that I have pictured here so that the user can choose more than one. Which should be simply changing this from current form choice to check marks but I can’t find that. I’m just not looking in the right place.

Also how would I add an entry to this menu where the user could manually add a name? Right now this is pulling info from a Smartsheet that would seem to be counter intuitive to how i believe this works.

You need to use an ENUMLIST type column for that. I’m not aware how you pull your data to this dropdown but provided you are using a SELECT expression in Valid_if, try using the that expression in Suggested_Values property and within the column structure of this column set Allow other values and AutoCompleteOtherValues properties to ON.