8 – How to set default value on Select2 Autocomplete via hook_alter

What’s the best way to set a default value to a Select2 autocomplete field widget on form load (I’m using a reference view to populate options). I’ve tried a ton of different variations from within form_alter with no success, such as…

$form('field_name')('widget')('#select2')('id') = 12

I can set the hidden <select> element with…

$form('field_name')('widget')('#default_value') = 12

…but this value doesn’t populate into the Select2 Widget.

I’ve combed through docs and issues without finding a solution. What am I missing?