Yes,
1st case, only one time after reboot,
everytime the machine reboot:
/system scheduler
add name=everytimeonreboot start-time=startup interval=0s \
on-event=":delay 20s;\r\n/system script run <SCRIPTNAME>"
2nd case, only one time after reboot, but
just for next reboot only:
/system scheduler
add name=onlyonetimeonreboot start-time=startup interval=0s \
on-event=":delay 20s;\r\n/system script run <SCRIPTNAME>\r\n/system scheduler disable onlyonetimeonreboot"
for execute again on boot, simply enable again the scheduler