First, if the 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 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 domain
Follow the next steps on the hPanel.
When it comes to changing the domain connected to your Horizons project, our team is working on implementing this and you can expect to see it live in your hPanel soon.
If such feature is crucial for you at this very moment, then we recommended the following alternatives:
Create a new project, connect the desired new domain and start from zero again
Delete the Horizons project to free the currently domain so that it could be connected elsewhere, if that's what the user wants
How to get your web app online
After connecting your custom domain to the web app 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.