You have two tasks to perform as per your explanation:
1) Get a confirmation from user that deletion of record is OK with him/her.
2) Record a reason for deletion.
Task 1 can be accomplished using javascript confirm box, code snippet given below:
function delete_confirm()
{
var r=confirm("This will delete the record. Are you sure you want to continue?")
if (r==true)
{
window.open("DeleteReason.aspx");
}
}