Populate Drop Down List

I simply need to populate a Drop Down List per an incoming Switch Value.

what is the syntax for adding a DROP DOWN OPTION to an existing list by ID?
to replace all in the list by the ID?

See skeleton below...

EXAMPLE HTML:

<select name="form_transaction_new.booking.medium_timing_id" id="form_transaction_new.booking.medium_timing_id">
 <option value="nan">Select Medium Timing</option>

</select>

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