How do I add an app to work with iCloud to back up data?

Greetings LorakiTX,

From our understanding, it sounds like you want to ensure an app that you've downloaded is included in your iCloud backup. If this option is available for that particular app, you'd want to go to Settings > Tap Account > iCloud > Show All. From there, you'd want to view the list of apps that can be included in your iCloud backup. It's important to note, that depending on the developer, some data may still not be included. So, if you have concerns about any particular data, you'll want to reach out to the developer to confirm using: How to contact an app developer - Apple Support

Once complete, you'd want to ensure an iCloud backup is created, since the app previously wasn't part of that backup. You can learn how to back up to iCloud here:

How to back up your iPhone or iPad with iCloud - Apple Support

"If you back up your iPhone or iPad, you'll have a copy of your information to use in case your device is ever replaced, lost, or damaged.

1. Connect your device to a Wi-Fi network.
2. Go to Settings > [your name], and tap iCloud.
3. 
4. Tap iCloud Backup.
5. Tap Back Up Now. Stay connected to your Wi-Fi network until the process ends. Under Back Up Now, you'll see the date and time of your last backup. If you get an alert that you don't have enough iCloud storage space to complete the backup, follow the onscreen steps to upgrade your iCloud storage."

You can learn what is included in that backup here:

What does iCloud back up? - Apple Support

Take care.