Declare a winner and end experiment

Declares the variant you specify as the winner of a <code>running</code> experiment, and then ends the experiment. If the experiment already has a winner (<code>winner_found</code>), the variant you specify replaces it. Iterable replaces the campaign's template with the winning variant's template. You can't declare the holdout group as the winner, and you can't change the winner after the experiment ends. This endpoint works only with experiments that test message content: the subject line, preheader text, from name and sender, email body, or all content. It doesn't work with send time or Send Time Decisioning experiments. This endpoint doesn't support journey experiments.