If you want user type some stuff in the multiline textbox, say ID="txtMulti", and you have the button with ID="btnPost". ASP 2.0 supports Panel control with DefaultButton property, you may want to include the multiline textbox and the button inside <asp:Panel>. So it looks like the following:
<asp:Panel runat="server" DefaultButton="btnPost">
<asp:TextBox runat="server" ID="txtMulti" />
<asp:Button runat="server" ID="btnPost" />
</asp:Panel>
This would be the easiest solution.
Or if you don't want to define a defaultbutton for some reason, you can do the following workaround for this textbox and button pair. First you need to ensure the masterpage's body enabled enterkey. Next is in the codebehind, inside a page event e.g.: Page_Load, add an javascript attribute to the textbox control:
protected void Page_Load()
{
// some code.....
StringBuilder sb = new StringBuilder();
sb.Append("if(event.which || event.keyCode) {");
sb.Append( "if ((event.which == 13) || (event.keyCode == 13)) {");
sb.Append( "document.getElementById('");
sb.Append( btnPost.UniqueID);
sb.Append( "').click();");
sb.Append( "return false;");
sb.Append( "}");
sb.Append("}");
sb.Append("else {");
sb.Append( "return true;");
sb.Append("}");
string js = sb.ToString();
txtMulti.Attributes.Add("onkeydown", js);
}