whenver selecting dropdownlist that value will be send to the server wihout clicking button in html5

You can write onchange event for dropdown list and in javascript make an ajax call with the data i.e selected value to the method or service i.e the server.

Consider the following example,

HTML:

<select id="ddl" onchange="sendToServer()">
    <option value="1">One</option>
    <option value="2">Two</option>
    <option value="3">Three</option>
    <option value="4">Four</option>
</select>

JavaScript:

function sendToServer(){
    var element = document.getElementById("ddl");
    var selectedValue = element.options[element.selectedIndex].value;
    
    //Make ajax call to send selected value to server
    $.ajax({
        url : "put service/method url",
        type : "post",
        data : {value : selectedValue},
        success : function(result){
                     //code block -- executes when the data is posted successfully
                  },
        error : function(xhr, status, error){
                // executes when any error or exception occurred in server
                }
    });
}