To secure your app name, go to PassionApps - Basics, type in your App Name, and click on the magnifying glass to check for availability.
If it's available, it will say "Congratulations!.." Click on the check button to save your app name.
Note: There are exemptions where Apple/Google will say that the app name is no longer available. In this case, nominate a unique app name and ideally, do a search in the app store to make sure it's not published by another developer.
If you would like to change your app name, click on the pencil icon to edit the name, and don't forget to click on save.
Watch this video with instructions on how to secure your app name as well as pointers: https://passionfighters.passion.io/learn/products/9365/lessons/91312