Do the Following Changes
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<appsettings>
<key value="Thank you for purchasing you can add new one <a href='NewItem.aspx'>New Item</a>">
</key></appsettings>
Create a DIV to show the Message.
<div runat="server" id="divSuccess">
</div>
Bind the Data From the Application Setting to Div Created.
divSuccess.InnerHTML=ConfigurationManager.AppSettings["Success"].ToString();