Hi, @ilyakonovalenko
The one that I tested was on the post that I created manually on the desktop app. Nevertheless, did you try to add the reaction using the cURL command I shared on your webhook post?
curl --location --request POST "<url>/api/v4/reactions" --header "Authorization: Bearer <personal_token>" --header "Content-Type: application/json" --header "X-Requested-With: XMLHttpRequest" --data "{\"user_id\":\"<your_user_id>\", \"post_id\":\"<post_id>\", \"emoji_name\":\"email"\"}