Add a string to a variable

I have the following code for a user bonnet:

<input id="DomainUserName" name="DomainUserName" type="text" class="textInputField" runat="server" size="25" autocomplete="off" />

In this way the user is obliged to enter domain @ user or domain / user, I do not want to enter the domain, that the domain receives the user and enter the domain alone.