If I understand you correctly, you want to associate the partner that is associated to the currently logged in user, to your delivery note. I see the delivery note is your page parameter.
When you create a delivery note, call a microflow. In this microflow, do a retrieve of the partner (via the current user). Then create an object DeliveryNote and associate the retrieved partner to this new delivery note. Then, open the NewEdit page.
As a sidenote, I would suggest making “Company” its own separate entity. This way a company can have multiple partners and you can easily retrieve them over company when needed.