Look for your database name on the main page of Atlas and click on the green CONNECT button. You should see a connection window pop up.
If this is your first time accessing the database, it will ask you to create a username and a password. Do so and keep that information handy. We'll need it at a later stage.
It will also ask you on the same page to "Add your current IP address". Click on the button and add 52.5.26.69 as our server's IP address.
Allowing 52.5.26.69 to talk to your database is necessary as HumongouS.io wouldn't be able to connect to your database otherwise.
Once that's done, click on "Choose a connection method" to go to the next page, and then click on "Connect your application".
Select "Short SRV connection string" and copy the SRV connection string.
Your connection string will look like the following:
Replace <PASSWORD> by the password you created earlier in the process, copy and paste your connection string into the project creation form in the app and click on Create project.