Hi @dvoinikat
I believe you have account lookup in your entity form or custom entity form.
Here, in my example, I have Appointment Basic Form As INSERT mode. In Appointment form, there is regardingobjectid field.
Step1: Go to your Basic Form and click on "Basic Form Metadata" Tab.
Step2: Click on "+ New Basic Form Metadata" button

Step3: In the Basic Form Metadata, Select Type as Attribute and select your lookup field where you want to populate the Company Name.

Step4: You will find "PrePopulate Field" section In the Basic Form Metadata.
Select Type = Current Portal User
From Attribute = Company Name (parentcustomerid) (Same as selected in screenshot)

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