See this answer to a similar question: https://community.atlassian.com/t5/Answers-Developer-Questions/Sprint-Velocity-using-Jira-Agile-REST-API/qaq-p/539237#M76894
TL;DR: It's possible, but not documented. You'll have to find the REST-URL manually by creating the report and watching the requests in your browser's devtools.
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