In order to autoload a zscript it has to be coded as a plugin and the plugin file (.zsc) has to be in the ZStartup\ZPlugs folder. The code for a plugin follows this form:
(The size of the button can be changed to whatever you wish.)
[ISubPalette, "Zplugin:My Plugins"] //creates a new subpalette in the ZPlugin palette
[IButton,"ZPlugin:My Plugins:Name Of Button",//note subpalette included in button name
"Popup text for button",
,,152,,,19]//button width 152 pixels, height 19
If you follow this method your button will load on start up and can be dragged onto the interface. Save the configuration (Shift+Ctrl+i) and it will stay where you put it.