How to secure a no-authentication game leaderboard?

I making a online quiz game (SPA web application) and I want to implement a leaderboard with the player’s score. I’m sending a request with the player’s initials and score to my back-end (REST API). Problem is that anybody could see the request to the REST API, in the network tab, and send a similar request from Postman. How do you secure such a leaderboard? There is no authentication – you just type in 3 symbols (your initials) just like in arcade games.

I’m using ExpressJS with MongoDB for the API.