You can use an Automated Action:

Python Code:

record['company_id'] = env.company

You might want to only apply this to customers by using Domain:

The effect of adding domain should be that if you create a customer from the “Customers” menu in the sales app then the company will be set. If you create contacts elsewhere it will not be set.

Use automated action to set company on contacts

Hello Clement, 

This kind of similar question has already been asked in the odoo forum.

Kindly review this one for ref. : https://www.odoo.com/forum/help-1/how-to-get-current-company-id-odoo-15-202859 

Thanks