Hello Milen
Unfortunately, currently we do not support $appsecret_proof.
I’ve created an internal ticket BKNDLSS-19554 for discussion, and I believe after that we will add the ticket to our roadmap.
But it may take some time for implementation and testing as well, so for now, if you can, just disable the “Require App Secret” feature on your FB App
We will notify you about any result here in the topic
Regards, Vlad