This isn't a technical question yet, since I'm still learning these technologies.

In the footer of their front pages, CompanyA.com and CompanyB.com would include a <script src="myservice.com/script.js"> referencing my code. For their customers, that script will generate a tiny UI and also connect to my nodeJs server.

Since Company A & B pay for my service, I need a way to ensure that company C can't simply copy&paste the same script. Also, I need to be able to identify from which company the connecting user came from.

Is this possible without any server side scripts in company A and B?