I'm afraid you are out of luck, insofar there is no REST API to achieve this, and the resp. issue status is 'not being considered':
So given there will not be any further feature development on Fisheye and Crucible since January 13th 2020, it is extremely unlikely that your vote or comment on that issue would make any difference.
Hi All! We’re excited to share the launch of an announcement banner that lets Jira site administrators communicate directly to their users across their Jira Cloud instance. ...
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