Now here's a POST request made by fetch in WebKit:

POST /v4/getposts.sjs
Content-Type: application/json
Origin: https://stibarc.com
Referer: https://stibarc.com/
Accept: */*
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: same-site
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/605.1.15 (KHTML, like Gecko)