TeXmacs has its own macro language. The equivalent of \newcommand is \assign together with \macro (yes, TeXmacs has proper first class macros, like any respectable language, e.g. Lisp). And macros arguments can be edited visually, here's an example: https://twitter.com/gnu_texmacs/status/1251554336842407938

you can check also the manual which comes with the program, in the menu Help->Manuals.

There is a good introduction in Chapter 12 of The Jolly Writer: https://www.scypress.com/book_info.html