For completeness, I should also mention 2 ways to install a macroscipt (.mcr) in max.
1. Place the .mcr file in the macroscripts folder which is typically:
C:\Users\[Username]\AppData\Local\Autodesk\3dsmax\2021- 64bit\ENU\usermacros
Then the next time you start Max, you will see it by name in the list of commands in the customize User interface dialogue. In the example I gave above, the command would appear as "Add_Human_Size_Reference" under the category "DragAndDrop".
2. More simply, you can just drag the .mcr file (not just the text, but the file itself) into a viewport in max and max will install it in the above macroscript directory. You won't see anything happen, but it will be available right away (you don't have to restart max like you would in method 1 above) to be selected from the Customize User Interface.
Just one tip: Sometimes the name of the .mcr file itself does not exactly match the name given in the Macroscript header of the file, so it is best to first open the .mcr file in notepad to see how it's name will appear in the list of commands in the Customize User Interface dialogue. Obviously you have the text of the entire macro in the current situation, but if in the future you get 3rd party .mcr files from somewhere, I thought I would mention it as I have encountered this.