Populate textbox from database using ajax

Hi, I have the following scenario: An SQL 2000 database with a table containing the columns UserID and UserName. A webpage with TextBox1 and TextBox2.

I need to use  AJAX  or Jquey to accomplish the following: When I type the patient ID  in txtPatientId textbox and press the Tab key, txtMRN will populate with the corresponding MRN number.

How do implement this this is what I  have but is not working.

[System.Web.Services.WebMethodAttribute(), System.Web.Script.Services.ScriptMethodAttribute()]
    public static string GetDynamicContent(string contextKey)
    {
        string outputString = string.Empty;
        string strConn = ConfigurationManager.ConnectionStrings[""].ConnectionString;
        SqlConnection dbConnection = new SqlConnection(strConn);
        
        SqlCommand dbCommand = new SqlCommand
        ("SELECT TPM300_PAT_VISIT.vst_ext_id AS VisitID,  FROM TABLE ......"

  return outputString;

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<asp:TextBox ID="txtMRN" runat="server" MaxLength="15"></asp:TextBox>
            <asp:DynamicPopulateExtender ID="txtMRN_DynamicPopulateExtender" runat="server" 
                Enabled="True" PopulateTriggerControlID="btn_Go" ServiceMethod="GetDynamicContent" 
                TargetControlID="txtMRN" ContextKey="txtPatientId">
            </asp:DynamicPopulateExtender>

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