I made a huge mistake in my article on how to post to Mastodon through IFTTT.
I gave the wrong directions on what to put in the Application Website field in Mastodon. It just needs to be
https://ifttt.com. It isn’t used for identifying the source to your Mastodon server as I thought it was, but appears as a link when the post is viewed by itself on the web. If you followed on my mistake, I highly recommend (1) removing the URL from your Mastodon config and just putting in ifttt.com and (2) going into your Webhooks settings at IFTTT and generating a new key. I feel horrible that I messed this up, and I am so sorry to everyone I steered wrong.