Here are some suggesions. This may help you to resolve your Problem.
1) You wrote -
"Now when user clicks Submit button, it should send en email reading the SendNotification()"
If you want to send Emails on Submit Button Click, Then why to Call
SendNotification()
method in
Application_BeginRequest
Event, Instead call it on your "SubmitButton_Click" event.
Remember
Application_BeginRequest
Event gets execute for every request comming to your Web Application.
http://aspalliance.com/1114_Understanding_the_Globalasax_file.all
2) You wrote
-"using Thread in background to avoid waiting to the user."
I would prefer Ajax for this, instead of Threading in Asp.Net.
Simplifying Asp.Net Core Ajax