Did some testing and it will populate a multi select field if only a single value is sent.
Does not work - ‘custom.user_field_1’ => ‘Pro, Newbie’,
Does work - ‘custom.user_field_1’ => ‘Pro’,
This is OK for our needs. If that field is editable by the end user they can add other selections as needed.
@Falco Thanks so much for the help