Pass key to Chrome extension (content.js)?

Question

JavaScript

Help me understand the question of how to pass the key to the google chrome extension.
there is an extension in content.js which has the following line:
key:"";
how can I pass the key to the content.js extension from a third-party site site.ru?
that is, in the content.js extension there is a line key:"";
then the user enters the site site.ru site, registers and after registering on the site in the content.js extension, the Key would be registered, which is issued after registering on the site