LexActivator is meant for Desktop based apps, and not for the single backend server. You typically have a single backend for all your customers, and only one license key can be activated using LexActivator in a single system for a product.
The only time you will use LexActivator for your backend app, is when you sell the backend itself, and each customer installs the backend on a different machine, which LexActivator will fingerprint and activate the license.
You cannot use protected Web API endpoints from the client-facing apps, as they require an access token. If someone gets access to your personal access tokens, your data gets compromised.
In your use case, you have to activate the frontend app, which can be done using Web API. You can use activation public API endpoints to create/update the activation and they return all the license data you need.
Refer to the following: