Hi @Zhi Lv - MSFT

Thank you so much for your response.

I need one advice from you,

I have slightly changed my code like this

    <button type="submit" title="Submit" class="btn btn-success btn-sm" onclick="return OnSubmitValidate()">
                            Submit
                        </button>
                        <input type="submit" id="BtnSubmit" asp-page-handler="Submit" style="display:none;" />


Javascript Function

    function OnSubmitValidate() {
            event.preventDefault();
            var RequestType = document.getElementById("DrpDwnRequestType");
            var RequestTypeValue = RequestType.options[RequestType.selectedIndex].value;
            
            if ((RequestTypeValue == "") || (RequestTypeValue == null)) {
                document.getElementById("ErrRequestType").innerHTML = "Please select Request Type";
                return false;
            }
    
            document.getElementById("BtnSubmit").click();
    }

Is this a right approach to follow?