Hi,
If you don't have a company field in your custom model then you have to add one like below
company_id = fields.Many2one('res.company', 'Company', required=True, index=True,
default=lambda self: self.env.company)
Then you have to write security rules regarding the visibility of the records
For example
<record model="ir.rule" id="sale_order_comp_rule">
<field name="name">Sales Order multi-company</field>
<field name="model_id" ref="model_sale_order"/>
<field name="global" eval="True"/>
<field name="domain_force">[('company_id', 'in', company_ids)]</field>
</record>
Above is the example of the model sale order. You can redefine with your custom model
You can also refer the following documentations
https://www.odoo.com/documentation/13.0/howtos/company.html
https://www.cybrosys.com/blog/how-to-handle-multi-company-odoo-custom-module
Regards