Hello,

here you need to override the create method of hr.employee.

def create(self, cr, uid, vals, context=None):
    user_obj = self.pool.get('res.users')
    vals_user = {
        'name': vals.get('name'),
        'login': default_login,
        #other required field 
    }
    user_obj.create(cr, uid, vals_user, context)
    result = super(hr_employee, self).create(cr, uid, vals, context=context)
    return result

Hope this will help you.

Thank you,

Serpent Consulting Services