In general you will need backend mechanics. There are three routes to achieve this:
1. Get a hosting server and write your own backend in php, mysql etc.
2. Get a hosting server and a self-hosted backend system (such as Parse Server).
2. Get a backend-as-a-service thing that handles all technicalities and lets you send/receive data using Construct's actions. Google Construct Firebase.
Route 1 can easily amount to more work than the game itself, and it will be very hard to to make it secure, reliable, scalable etc. so you can skip it upfront. Route 2 will also require skill and then you will need to use third party plugins or JS. So the best option is to use route 3 to create an online database for storing users and then create a system of sign-up/sign-in within your game.
Just a heads up: this is an undertaking that still can prove more cumbersome than coding the actual game.