There is already one example:

https://stackoverflow.com/a/78990144/22768315

The point is that the string must follow exactly the following scheme:

"[FAMILY-LIST] [STYLE-OPTIONS] [SIZE] [VARIATIONS]"

Further explanations can be found in the documentation of GTK4:

https://docs.gtk.org/Pango/type_func.FontDescription.from_string.html

Have fun testing.