show hide field when picklist value

Hi,

Our customer has a picklist field "type_c" on account with value "company" and "person".

A field "companyid_c" is visible when the value "company" is selected, otherwise this field is hidden.

They want me to change the functionality to: Always visible, but required when company is selected.

But I can not find how the visibility rule is configured. In studio there are no formula's and in files/code I can't find where it is configured.

Any tips on where to start? What file is usually edited to achieve this?

Thanks,

Stijn