Try to set icon type as "Project" and add Default tool "c:iconx". It normally works from Ambient, try if it works also from WBstartup.
Or, simply add your sript to s:user-startup. It will work for sure, but I know this is not the exact you want.
AmigaOS: Micro A1-C, AmigaOne XE, Pegasos II, Sam440ep-flex, AmigaOneX1000
MorphOS: Efika 5200b, Pegasos I, Pegasos II, Powerbook G4, Mac Mini, iMac G5, Powermac G5 Quad