Hi,

Here in the button click you are returning maintenance.request model with some default values inside the context, so for the record to be created you have to click the Save Button. As you need to create the record without clicking the save button, what you can do is that, create record in the maintenance.request model from code and return the created record id in the return.

@api.multi
def on_assigned(self):
    vals = {
        'helpdesk_id': self.id,
        'name': self.name
    }
    # you can update above dictionary with other values and required fields
    new_rec = self.env['maintenance.request'].create(vals)
    return {
        'name': _('Maintenance Request'),
        'view_type': 'form',
        'view_mode': 'form',
        'view_id': self.env.ref("maintenance.hr_equipment_request_view_form").id,
        'res_model': 'maintenance.request',
        'domain': [('customer_id', '=', self.customer_id)],
        'context': {
            'default_helpdesk_id': self.id,
            'default_name': self.name,
        },            'type': 'ir.actions.act_window',
        'res_id': new_rec.id,
    }

Thanks