This duplicates an identical question asked previously on SO.
However, the accepted answer there appears no longer to work. The accepted answer there is:
Prior to this I had only a CNAME Record with a 'www' host pointed to Heroku. Now I also have an ALIAS Record with the root host '@' pointed to Heroku.
I tried to add an ALIAS record on Namecheap with host=='@', pointing to 'https://www.example.com'. But, Namecheap put up a message saying:
Error occured: INVALID_ADDR : 'https://www.example.com' should not be an IP/ URL for ALIAS record.(host name: @)
I also tried an ALIAS record with host=='@', pointing to my web host (us-####-#.galaxy-ingress.meteor.com). Namecheap accepted that, but my web host said the Namecheap configuration was incorrect when I tried to generate an SSL certificate.
What's the correct way to do this on Namecheap?
At the moment Meteor Galaxy requires a CNAME record, and as noted in the comment above, Namecheap doesn't want you to use a CNAME record for this. The solution, provided by Galaxy, was to use a third-party service. I won't post the url to the 3rd-party service here, as that might look like spam, but you can read about it in Galaxy's docs here: