How to add label to Pull Request via Github API?
I made it work by changing the way labels argument is passed into requestBody.
Turns out the list representation while interpolated into string is missing quotation marks around each list element.
def setLabels (String repository, List label) {
    try {
        String labelsAsString = &quot;[\&quot;${label.join('&quot;, &quot;')}\&quot;]&quot;
        def response = httpRequest httpMode: 'POST',
                        url: &quot;[REPO_URL]/${repository}/issues/${CHANGE_ID}/labels&quot;,
                        authentication: 'ors_git_service_account',
                        requestBody: &quot;&quot;&quot;{
                            &quot;labels&quot;: ${labelsAsString}
                        }&quot;&quot;&quot;
    } catch (e) {
        wrap([$class: 'AnsiColorBuildWrapper', 'colorMapName': 'XTerm']) {
            echo &quot;\u001B[31magdSlack.setLabels: '&quot; + e.toString() + &quot;'\u001B[0m&quot;
        }
        throw e
    }
}