Help in Hidden Field

Hi,
Please help me why i'm receiving this error:
The name 'isTest' does not exist in the current context
 
<form id="formInfo" action="">
            <input type="hidden" name="Test" value="<%=isTest%>" />
           
</form> 
 
 protected void Page_Load(object sender, EventArgs e)
    {
        string isTest;
        isTest = "Hello World";
        TextBox1.Text = isTest;
      
  
    }
 
How can I pass value to the hidden field? Thanks.
 
 
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