As far as I can see, https://github.com/hammerfunctor/tm-mathematica is the only way to use Mathematica inside TeXmacs on Windows.
I downloaded the zip file to the TeXmacs plugin folder on Windows and renamed it to mma as stated in the instructions on GitHub.
Moreover, I ran the menu entry Tools > Update > Plugins.
Running
(url-exists-in-path? "wolframscript")
returns #t, indicating that TeXmacs can find the code.
However, there is no menu entry like Mathematica in the Insert > Session menu.
Maybe @sharkc has an idea which additional diagnostics I could run to pin down the problem.
I carefully read http://forum.texmacs.cn/t/texmacs-unable-to-run-mma-mathematica-plugin-in-mac-m2/2015, but this shouldn’t affect Windows.