Ok, following this thread: Create issue via REST API - what shoudl fields / update data look like? - The Atlassian Developer Community
I got to this page:
REST APIs (atlassian.com)
Looking at examples there I finally make it work with following structure:
{"update":{"components":[{"set":[{"name":"engine"},{"name":"backEnd"}]}]},"fields":{},"transition":{"id":"51"}}
Is there a page that describes fields input formats for cloud as there is for server?