How to interact with the extension?

Question

API

I'm trying to make an extension that will save information on my site when a certain button is pressed, then insert this information on another site when another button is pressed there.
How to save information to the buffer, I can’t find the information ...
How to paste it too, I wrote a short test

document.addEventListener('DOMContentLoaded', function() {
var checkPageButton = document.getElementById('checkPage');
    checkPageButton.addEventListener('click', function() { 
               document.getElementById('test_1').innerHTML='test'; //если блок
               document.getElementById('test_2').value='test'; // если поле

    }, false);
}, false);

this id is in popup.html and it will display it there, but if there is a field on the page of the desired site with the same id, for example, then it gives me an error
Uncaught TypeError: Cannot set property 'value' of null
Uncaught TypeError: Cannot set property 'innerHTML ' of null
In general, if anyone knows how to set values ​​​​through extensions for third-party sites, as well as save info to the buffer, I will be glad if you share, or if anyone has links to similar topics...