Hi,
We can retrieve the last created record in the model “back.label” using search method by specifying the order and using limit parameter.

back_label_id = self.env['back.label'].search([("domain")], limit=1, order='id desc')

Hence you will get the last created record object in back_label_id.After getting the object you can assign the value accordingly.If you have any domain used for the search operation please specify the domain in the brackets also.

Eg:

def _get_default_back_label(self)
    back_label_id = self.env['back.label'].search([("domain")], limit=1, order='id desc')
    return back_label_id

Regards