The purpose of the 18 tooth variant is to support low Watts at a reasonable cadence. Some background:
If that’s important for you, and the 18 tooth Cog isn’t available, you could also consider using a third party single speed conversion kit with 18 teeth or more. Here’s an example that’s compatible with 7-11 speed chains. There are some available that claim 12 speed compatibility as well. I recommend getting one with a steel cog rather than aluminum.
Of course that leaves you without the Click shifter, but if your bike has drop handlebars I recommend getting the Play controllers instead since they do more things.
The cog is also not required for virtual shifting - you could use a normal cassette, unless you need to swap between two bikes with different drivetrain compatibility.