Startup-Sequence is taboo, don't touch it ! The correct place for customisations is User-Startup.
However, mounting a share is better synchronised with the TCP/IP stack because trying to mount it before TCP/IP is online will fail. Depending on the stack you use there are different places to add scripts to events. Check your TCP/IP prefs.
According to my experience, you better delay the mounting of shares a little bit after going online. So to the online event add something like "run >nil: execute s:online_script" and in s:online_script you do this:
wait 1
mount myshare:
Bye,
Thomas