Install your app
You can install an app into a subreddit you moderate ('everything' permissions) using either devvit install or through the Developer Portal.
Install with CLI
Once you’ve uploaded your app, you can install it on a small subreddit that you moderate (less than 200 subscribers) by running the following command from the project folder.
$ devvit install <subreddit-name>
Install with Portal
To add an app to your subreddit, find it in the Developer Portal. Click on the app info card to get to the App Details Page, and click "Install".
This will give you a list of subreddits that will allow you to add the app. Generally, you can only add an app to subreddits that you moderate.
Select a subreddit to install the app.
If the Install button is not active, you are not the moderator of an eligible subreddit. Try creating a new test subreddit with fewer than 200 members.