Textbox submit after pressing Enter

I've got a simple textbox and a imagebutton.
If i press enter in the textbox, i like to run the code of the imagebutton onclick.
But why is there no keypress mode ore someting like that, so i can check if the user presses the enter button?
So how can i solve it?
 The code is quite easy:
<asp:TextBox ID="txtSearch" runat="server" CssClass="search"></asp:TextBox>
<asp:ImageButton ID="btnSearch" runat="server" ImageUrl="Images/button.png" onclick="btnSearch_Click" />

So if the user presses enter in the txtSearch, he should run btnSearch_Click.
Why is this so difficult ? On the internet there were all javascript methods for it, but why not in ASP.Net ?
Thanks.
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