String builder append issue ?

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C#3.5

I have one POST variable "DEST", If post variable is null then I need to pass hardcoded value "app" .In Redirecttopage() method I have string builder "sbSourceString ".i am appending "sDest" and "userid" to this "sbSourceString ". I have issue while appending the variable. please find the sample code.
protected void Page_Load(object sender, EventArgs e)
{
string sDest = string.Empty;
if (!string.IsNullOrEmpty(Request.Form["DEST"]))
{
sDest = Request.Form["DEST"];
}
else
{
sDest = "app";
}
Redirecttopage();
}
private void Redirecttopage()
{
StringBuilder sbSourceString = new StringBuilder();
//sbSourceString.Append("dest=app&UserId=07-");//earlier code

}
so how to append my sDest to the string builder .

Please help guide me.

What I have tried:

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