Changing a variable from an input box on web page

Hi and thank you for checking this out. I am working with an esp8266. I have it connected to my network as a client, and I can bring up a page in the browser using the ip address.

The esp is also running aREST code to connect to their It all works. I just want to change my aREST key from a web browser page that I can call from any device connected to the same network that my esp is on.

I have a page that I can call from the esp that will give me a text box and submit button. How do I get that text (new updated aREST dashboard key) into where I coded the aREST key in arduino IDE?

Thank you!