how to secure api server

I am developing my api server at the moment, but one problem comes to my mind which I am not sure how to approach.

Since the api server runs on port 3030 and I can make requests to this api server from postman, how should I prevent requests coming from not-my-client server. I only want to listen to requests from my app which is running on port 3000. This is not a problem for now, but in the future I don t want anybody else to make requests to my api server rather than my client server.

Is there any solutions or am I missing something ? Thanks for your help.