field like "Current Employer" that creates new organization and relationship
If you want a custom "Contact Reference" field to always create a relationship whenever it is saved, you would need to add some custom code implementing hook_civicrm_post.

However, before you go down that road I suggest you clarify why you want to create both a custom field and a relationship to store the same data. Is it just because that's what the "Current Employer" field does? Not a great reason in my opinion. It's generally a bad practice to store the same data in two places, so the overly-complex "Current Employer" field is not setting a great example of tidy database practices.

I suggest you decide to use a custom "Contact Reference" or a custom relationship type, and not both. Trying to store the same data in 2 places is rarely worth the trouble it causes.