How did I do this, linking text boxes?

Sorry for the cryptic title. I am getting older and my brain is fried! For many years I have used a template which I created, I have just adjusted as needed but I want to start from scratch and dont remember how I got here.

Essentially I have a text box that determines what will go into the title of a page. The purpose of this is so that title remains page after page until I change the text box again on the next page to be a different title. This stops double ups in the contents page

In the image below the title text has arrows at each end and is determined by the 'Contents' text box.

If someone could just give me a hitn I am sure I can work it back out!

In an hilarious twist of fate I asked this question a few years ago but apparently I worked it out and did not write it down lol.