Hi Hunain,

The default Odoo Formbuilder does support many2one fields. You can add it by first dragging the "Form Builder" on the page and then clicking on "Addan existing field" from which you can select existing fields.
An example with the countries list for contacts:

At this point the Form Builder can only access/use fields depending on which action you want the Form builder to do though (so create a contact, lead, ...).
If you want it for another model such as user or a custom module you will need to extend the Form Builder widget so it supports your models too.

Regards,

Yenthe