Thank you, that worked fine. In the past when I first opened a new release, macOS asked if I really wanted to open the app. This time I was confused because the first message flat refused me. On the second try though, the message had an option to open the app.
As an aside, I clicked on Donate on
https://sigil-ebook.com and it says:
Not available at this time.
Living on SS, I don't have much, but I thought it worthwhile.