You can do 2 things,
Put this on your Perform on Connect ( ALT-O -> CONNECT -> Options -> Perform -> Check the box "Enable perform on Connect" -> select Undernet)
.msg X@channels.undernet.org login USERNAME PASSWORD
mode $me +x
Or Add an On Connect event in your remote section
On *:CONNECT: {
if ($network == Undernet) { .msg X@channels.undernet.org login USERNAME PASSWORD | mode $me +x }
}
**Change USERNAME & PASSWORD