I created this pattern to recognize Letsencrypt (acme-protocol) challenge.
You need to create a custom application with these fields:
That's the best I could bet.
Btw, I did not know before that "http-req-uri-path" had to include the method (GET), so I had a hard time using the regex anchor "^".
You can look at the "QUALIFIER" option as you can select the request method "POST" or "GET" from there and maybe you will not need to specify it in the pattern match.
Also for blocking something maybe better use IPS signature than app signature:
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