Hi partner,
So the organization is a custom entity in your D365 right?
If so, I recommend you to add an organization look up field on your account form, and then go to Dynamics 365 portals->Entity Form, find your account create and edit forms, new an "Entity Form Metadata".

Select type to Attribute, and choose your organization lookup field, in my example I used the customer field on case form.
And choose the Control style you need.



Best Regards,
Leo