Hi， Josef，

you should define the many2Many field with the default option.

service_ids = fields.Many2many('sale.order.service.type.line', 'service_type_line_tag_rel', 'order_id', 'order_service_type_id', string = 'service_ids', tracking = True, default = lambda self: self.env['sale.order.service.type.line'].search([('id', '=', 2)]))

Attention to the default option in the above example and modify it to your own requirement.

Thanks.

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Update:

Josef， for details you can refer to:

https://www.odoo.com/zh_CN/forum/bang-zhu-1/how-create-a-check-box-field-in-custom-module-94390

Thanks.