First, if the main domain is not pointed to Hostinger, make sure to change the nameservers to those of Hostinger and then proceed. You can find more details about it here. You can also point it following a second method, which would be via A records, also stated in the article.
If the main domain is already registered at Hostinger or pointed to Hostinger, proceed to the next steps.
Navigate to the Websites list section on the left sidebar,
Click on Change domain for the specific web application that you wish to connect to your subdomain
Follow the next steps on the hPanel and add your subdomain:
How to get your webapp online
After connecting your subdomain to the webapp created with Horizons, in order to get your web application live, you need to deploy your project.
To deploy it, click on the Deploy button in the top right corner of your Horizons tool:
When the deployment is successfully completed, you will be presented with a unique generated link to view your new web app:
When the deployment is successfully completed, your new web application is now live and ready to be used 🚀
You can find more details about deploying your project in this article.