What should I put in Host Name when buying a DNSimple SSL certificate for it to work with the Heroku ssl addon?

I'm confused with this line in the Heroku docs (https://devcenter.heroku.com/articles/ssl-certificate)

You must enter a subdomain in the “Host Name” field. Leaving it blank will generate a root-domain certificate which is not compatible with Heroku’s SSL endpoint.

What I want is https://foo.com and https://www.foo.com to work.

So what do I need to fill in this field to get a SSL certificate that will help me achieve the above?

The rest of the steps (setting CNAME to Heroku's SSL endpoint, adding an ALIAS to redirect the root domain etc) are clear to me. I'm just stuck on this step, should it be blank or 'www' or something else?

If you want your certificate to be valid for the rood domain AND the www hostname, then you should use the www.example.com version.

You can also purchase a wildcard, but unless you need to support any extra subdomain, the cost is not worth in this case.

More details are available in the support page Selecting the Certificate Hostname.