What is the syntax for changing a user with a macro?

The easiest way for this is to learn the macro while using the user selection in the bottom right corner of your main display. The resulting macro will say "User_ID 1 * ".

Just a note about your specific situation. Sometimes lighting designers want to be in your user so they can read your command line. To achieve this you don't have to use the same user number, you could give your LD also a magic sheet that has your command line.

Thank you so much! The designer requested it because they want the channels I select to be selected on their monitor as well (meaning they wanted channel 1 to appear whenever I typed [1][Enter]). If there's another way to do that I'd be thrilled to know

If you want to show them what you are doing and have them in a different user, you could put your command line on a magic sheet for them. Just put a command line object in and choose your user as the target. 

The designer's computer is in mirror mode, so he can see everything the programmer does (screens, channels, commands...) ...he only monitors the programming...

Often lighting designers just want you in their user so they can see the selection outlines in the channel display, it would be great if you can have that tab display the selection of a different user.

It's been a while since I have browsed the new features request forum, but I am heading over to up vote that one

Always appreciate support in the feature requests! Some others you might like:
Highlight timing
Show update target in command history
Add downloads folder to quick selection in resource browser

A similar feature request is also in the forum:

 New Flexi states - Selected By User X, Selected by other users 

That is also a great idea.