How to embed group name on the header?

I successfully made invoices to my customers, using the following post directions.

https://community.fabric.microsoft.com/t5/Desktop/Single-Company-Data-in-each-page-using-Power-BI-Report-Builder/td-p/2389714

But my customers purchase details are too many to fit on one page.

There are 2 or 3 pages at each customer.

So, I want my report header that has current customer name.

How can I do this?

I tried

=First(ReportItems!Tablix2.Value)

(or something like this) on the textbox in the header, but it does not work.