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string [] issueKeys = selectIssues("project = Test");

for(string key in issueKeys) {   
    %key%.labels = "newLabel";
}

When updating the values of fields using a dynamic context the issue key must be wrapped in % signs. This shortcut converts the issue key (which is simple text) into an editable issue object. Without using the substitution shortcut (the % signs) the issue is read-only.