The startup.scp script has moved in version 1.8. It is now on the etc/startup installation directory, so the correct command to edit it is as follows:
open "$PXI_BASEDIR/etc/startup/startup.scp"
The rest of the information in the link Steve has given above is correct. For a normal PixInsight installation you'll have to have administrative rights to change this script, as it is on a protected system directory (such as "C:\Program Files" on Windows or "/opt" on FreeBSD and Linux).