How to connect phone and web application

I'm working on a project and i want to connect android app to my website.

Say i have an app running on my android phone that scans NFC tags nearby. I also have a website running with an input field.

Now I want the scanned value in the app filled in on the website input form instantly/ automatically. Or a webpage say 'waiting for scan'.

I know how to create both. So a web app and an android app. I just need to know how to connect those two.