Looks like you can retrieve the notification text here:
https://api.twitch.tv/kraken/users/:USER_ID/notifications/custom?notification_type=streamup
{
"user_id": "13074",
"notification_type": "streamup",
"message": "This is a custom go live notification",
"is_default": false
}
It’s Kraken v5 so make sure you substitute the user ID.
<general disclaimer about it being an undocumented API>