FYI: I opened a WiiM support ticket for this, and WiiM confirmed that presets cannot be initiated by voice command.
Although
this WiiM FAQ says one can say "open preset one" to have the WiiM start playing preset #1, as of 1/9/24 WiiM confirmed that, unfortunately, that won't work. It was unclear whether it's being worked on, or if there's a limitation with Alexa that makes it infeasible...