Thanks to Jean Abou Samra I can now create \markup that goes to a place in a youtube video.
These are of the form:
fis4^\markup \with-url#"https://youtu.be...?t=SEC" {ref} where "SEC" is replaced with the number of seconds in from the start.
I would like to create a function to simplify things so instead of having the entire URL repeated everywhere, would have something that looks like
fis4^\mymk "215".
So far, I remain entirely confused about creating a new markup command. I won't bother describing my many failed attempts. I am clearly missing some basic concepts. I have no problem creating scheme functions to do various manipulations, but extending the markup function has eluded me.
Any guidance would be most appreciated!