Unfortunately that is easier said than done on Azure. - I’m the author of the site extension, and MS is trying to help with a way forward, but it is gonna take a little while (most likely a few weeks). So if your site is a production site where it is a must to have a fully working SSL you will have to go buy a certificate. I recommend that you follow along in this issue in the site extension github repo. https://github.com/sjkp/letsencrypt-siteextension/issues/42