how to access HTML control from code behind

Hi;
my application runs on the client side.  I have to access a display on the asp page via javascript.  I have the following code working properly:
ClientSript.RegisterStartupScript(this.GetType(), "allMessages", "<script>document.forms[0].tbDisplay.value='Messages'; </script>");
tbDisplay is a "text box" asp:textbox control.  I need to replace de 'Messages' for avariable.  so instead of printing "Messages" all the time, I can change the information on the variable prior to display the message.  Any idea how to change 'Messages' to a variable.
Thank you 
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