For this to work, you need to have your secondary domain connected to a hosting plan where you can access files via FTP or from the hosting interface.
Make sure you don't change any DNS records other than A record.
Steps
1. Remove default domain forwarding
2. create an HTML index file that will redirect your secondary domain to your primary domain. You can use the form below
3. Change the existing index.html of your secondary domain with the new index.html file you got above.