I was able to figure it out. I found the WMTS GetCapabilities document. I found it by clicking this:
at the top of this page (for example). JOSM’s documentation states that, for WMTS, all it needs is the link to the GetCapabilities document, and it’ll then configure it on its own. Furthermore, the link that I had was for a MapServer (it has Esri documentation here), and under that, it states that it supports WMTS.