How to add Company name as a field on a Sign up User Flow?

Using Azure ADB2C Identity server.

How can I add a field for the "Company name" user attribute in a sign-up flow?

I see that Company name is a built-in attribute of a User in Azure ADB2C:

I already have created a working Sign up flow that is populating the First name, Last name and Job title attributes:

I would like to add Company name to the Sign-up form but I do not see the Company name as a "Built-in" user attribute available out of the box:

Should add a custom attribute? How to link it correctly to the built-in "Company name" user property?

Not all the fields shown on the screen are B2C user attributes.

As per this, "Company name" is not an attribute.

You could add it as an extension attribute.