I can't give you exact details as I don't do ASP.NET, but here's a few resources:
To send a request from asp.net:
Call external json webservice from asp.net C#
An example dwolla API endpoint:
https://developers.dwolla.com/dev/docs/transactions/send
Your post request would look something like:
POST https://www.dwolla.com/oauth/rest/transactions/send
Content-Type: application/json
{
"oauth_token": "",
"pin": "",
"destinationId": "",
"amount": "",
"destinationType": "Dwolla",
"assumeCosts": "false",
"additionalFees": []
}
Of course, to do this you would need the user's api token, and they'd need to enter their pin.