Each button that can be created using the Custom Buttons plug-in is a python script. To save the script, click on Save or Save as and specify script name.
It is very important that the scripts must be located here
c:\Users\%USERNAME%\AppData\Roaming\MAXON\CINEMA 4D (your version)\library\scripts
In this directory, you can distribute scripts to subdirectories. In addition to the script file, the plug-in creates additional files containing preset data. Please, don't delete or move it. Without these files, the button will not work.
After saving, you'll need to restart Cinema 4D. To do it quickly just open plugin window menu Edit > Restart Cinema 4D. The saved script button will be available in the Script > User scripts Cinema 4D menu. For convenience, you can place it on the toolbar or assign a hotkey.