The URL is unfortunately not working. Please note that this falls under the customization request category, which is not part of the support process, so I cannot guarantee it's going to work at all.
It's probably not working because it's name is not unique. The input id and name is “set_inexcerpt” and the label is for the “set_inexcerpt” field as well. You should probably change that as well.
Please note that by simply adding a filter to the front-end will not make it functional. A handler must be created for the back-end as well, which is not an easy task to do without knowing the code. The option will be there, but it won't do anything at all.Best,
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