how to trap a Enter Key from the Server Side

Good Day all

i have a dummy javascript that can trap for enter key on the client side

function Enterkey()
    {
        var textb = document.getElementById("<%=TextBox1 %>");
        if(window.event.keycode ==13 && textb.value.length > 1)
        {
         alert('Something will be Returned');
         alert(textb.value.length);
        }
       return false; 
   }

and on my server side , i am trapping for textlength with something like this

  if (TextBox1.Text.Length > 1)
        {
            //Do whatever 
        }
        elseif(TextBox1.Text.Length > 1 && "And the user Pressed Enter")
        {
            //do whatever 
        }

now look at the Elseif part, if the textbox is greater than and the user has pressed the return key(Enter) then it should execute the do whatever condition. i could do this on the client side , but my app requirement has changed so some of the code is on the server side.

how can i achieve this

Thanks

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