Populate textbox from database using ajax

[WebMethod]
  public static string GetMRN(string PatientID)
  {
        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 ......WHERE .... = " + PatientID      
// ... get MRN

        return strMRN;
   }

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<script type="text/javascript">
function getMRN() {
    var PatientID = $get('<%= txtPatientId.ClientID %>');
    if(PatientID.value == '') return;
    PageMethods.GetServerData(PatientID.value, OnSucceeded, OnFailed);
}
 
function OnSucceeded(result, userContext, methodName)  {
    var MRN = $get('<%= txtMRN.ClientID %>');
    MRN.value = result;
}
 
function OnFailed(error, userContext, methodName) {
   alert(error.get_message());
}
</script>

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<asp:TextBox ID="txtPatientId" runat="server" MaxLength="15"></asp:TextBox>
            <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" 
                ErrorMessage="*" ControlToValidate="txtPatientId"></asp:RequiredFieldValidator>
            <asp:Button ID="btn_Go" runat="server" onclick="btn_Go_Click" Text="Lookup" 
                UseSubmitBehavior="False" CausesValidation="False"        />
        </td>
        <td>
            MRN:
            <asp:TextBox ID="txtMRN" runat="server" MaxLength="15" ReadOnly="True"></asp:TextBox>
            <asp:DynamicPopulateExtender ID="txtMRN_DynamicPopulateExtender" runat="server" 
                Enabled="True" PopulateTriggerControlID="btn_Go" ServiceMethod="GetDynamicContent" 
                TargetControlID="txtMRN">
            </asp:DynamicPopulateExtender>

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[System.Web.Services.WebMethodAttribute(), System.Web.Script.Services.ScriptMethodAttribute()]
    public static string GetMRN(string PatientID)
  {
        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, TPM300_PAT_VISIT.med_rec_no AS MRN  WHERE  TPM300_PAT_VISIT.vst_ext_id = '" + PatientID + "'", dbConnection);


     -->>>>   return strMRN;?????????????

   }



}

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SqlDataReader rdr  = cmd.ExecuteReader();
String strMRN;
if(rdr.Read()) {
    strMRN = rdr[1];
}
return strMRN;

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