Entity is called internal_organization in forms. To get to the internal_organization record, you would do object_name.internal_organization in the custom form.
For example, for order related forms (sales order, purchase order, warehouse copy), you get access internal organization with this order.internal_organization.
A common use case is for you to get the name of the internal_organization. You can output the name of the company of internal_organization like this
Or get address like this
For more details on internal organization, see this.