This only works with properties (new function in ArchiCAD20), as far as I know. So you need to set up a property for the model name and allow that to take text as input.
Here are a lot of in depth videos about properties and how they work https://www.youtube.com/playlist?list=PLnXY6vLUwlWWXsVJ3fRGNxuKLoHPkpx1I
It is a lot to explain, I had a 4+ hour update course for ArchiCAD20 with our reseller to cover all the new stuff in ArchiCAD20, and this is definitely a big update feature.
Erwin Edel, Project Lead, Leloup Architectenwww.leloup.nl
ArchiCAD 9-26NED FULL
Windows 10 Pro
Adobe Design Premium CS5