@dipro Regarding the problem experienced by @simonWT I noticed some confusion in the cURL example in your documentation at https://monday.com/developers/v2#introduction-section-how-to-get-started-using-the-api.
The problem is that while query command works both if wrapped or not inside another query property, mutation requires wrapping.
{"query":"query { ... }"} → WORKS
{"query":"{ ... }"} → WORKS
{"query":"mutation { ... }"} → WORKS
{"mutation":"{ ... }"} → DOESN’T WORKS