You are very restricted in your style options with the online version, and I don’t know whether you have an institutional administrator who can add style or what styles you have available. I also don’t think that the journal terms list can be applied using the online tools.
Desktop endnote has that style and says it is based on the AMA Manual of Style 10th Edition (Endnote may call it JAMA). Is that style available to you? Unfortunately it appears that the style lists 3 authors et al when there are 7 or more and not 3 or more, but in my experience a journal can make this adjustment in house - or you could find a friend with a desktop version of endnote and download and edit the style for the final version you want to submit. Even the desktop version of Diabetes Tech Therap has this setting incorrect. I fixed it in the copy attached (in a zip file), if you have a friend with the desktop version.
Diabetes Tech Therap.zip (3.6 KB)