samuelsouza
You would first have to create a Custom List and manually define each Organization as a List Item. Then you can create a Custom Field on a Custom Form (or Ticket Details Form) and set the field type to the Custom List you created.
Keep in mind that this doesn’t affect the actual Organization in any way; it is a completely independent Custom Field. There is no way to use the actual Organization as a field type unless you modify the code yourself.
Cheers.