following cutyom code adds a stock pick:

picking = self.env['stock.picking'].sudo().create({
    'location_dest_id': output_location_id,
    'origin': 'Leihgerät-Rückgabe von ' + str(self.sale_id.name),
    'location_id': 9,   # Partner Locations/Customers
    'picking_type_id': picking_type_id,
    'partner_id': self.partner_id.id,
    'group_id': group_id,
})

group_id is the id of an existing procurement group.

but after creation of the pick the group_id field in Database is still empty.

how can i connect the manually created pick to an existing group_id?