Can't communicate from docker container (.NET Core App) to postgreSQL on host droplet!

What connectionstring should I use to connect from my Docker container (.NET Core App) to my PostgreSQL database installed on the host Droplet. Both container and database are on the same droplet. Navigating to the website works but get error:

Failed to load resource: the server responded with a status of 400 (Bad Request)

When trying to communicate with database.

This is my connectionstring:

  "ConnectionStrings": {
    "DefaultConnection": "host=;database=my_db;username=username;password=password"
  }

I’ve tried both public IP and private IP for host part in connectionstring but doesn’t work.

What should be used here?