Hi team,
If anyone from Atlassian sees this.
We got stuck for a bit in this part of the documentation.
We spend a bit of time trying to troubleshoot and testing alternative way to make this call.
“Request bodytext/plain” - this part should be application/json
Also an example in “List of issue keys“ like [PROJKEY-1, PROJKEYB-2, PROJKEY-2] would be top notch. 
All the best,
Vasco