(I am copying this from a post by CrazyGecko to a thread of its own, and perhaps it can be made a “sticky” by a forum moderator).
UPDATING May 2018
I note there is a webpage for Journal editors and librarians to submit requests for new and updated styles here (but not for individuals). They suggest you ask the publishers to request here.
Endnote online: Creating new output styles or modifying existing styles
You can also check with our EndNote Community Forum here:
You can use this forum to submit your request for the creation of a new output style (or the modification of an existing one). When you submit this information, your request is brought to the attention of our Content Group. Once the style is created or updated, it will be submitted for inclusion into EndNote online.
The local EndNote online Administrators also have the ability to update Styles for their site if EndNote online access is provided by Web of Knowledge. If you have EndNote online access through your institution, you can contact your local Administrator to update existing Styles or upload new Styles. (this information is from here) The instructions for the Administrator are here.