Hi， there,

the following for your reference:

1. modify the .py file to add one function.

def preview_inventory(self):
    return {
        'name': _('Inventory Lookup'),
        'domain': [('po', 'like', self.po), ('partner_id', '=', self.partner_id.id), ('company_id', '=', self.company_id.id), ('state', '=', 'done'), ('picking_type_id', '=', 7)],
        'view_type': 'form',
        'res_model': 'stock.picking',
        'view_id': False,
        'view_mode': 'tree',
        'type': 'ir.actions.act_window',
        'target': 'current',
        'context': "{'toolbar':False,'submenu': False,'create': False,'edit': False,'delete': False}"
    }

2. modify the. XML file to add one button in right place.

expr="//div/button[@name='preview_sale_order']" position="before">
    

Replace the element with your own.

I hope it is useful.

thanks.