Idea:I am automating wifi login for a particular network where user have to enter credentials by poping a toast notification like alarm app but instead of snooze or dismiss butt
You need to use a Background Task which is getting triggered by a System Event - in your case the NetworkStateChanged
trigger.
Here are a list of available triggers:
SystemTriggerType Enum
And here is a quick introduction how to respond to such a system event:
Respond To System Event
You can then check if you want to show the toast by using Windows.Networking.Connectivity.NetworkInformation.GetInternetConnectionProfile()
Look at this question https://stackoverflow.com/a/32846558/5111904 or look up the documentation of the NetworkInformation class for further information.