how to auto update reason based on request No using javascript ajax

I work on asp.net mvc i face issue i can't auto update reason using java script ajax 

auto update meaning when user start writing on reason it will save on database table 

update i need will done without using update button it will auto save using java script ajax or jquery

my html as below 

@model HR.WorkforceRequisition.Models.ResignationRequester 
<table style="border: 1px solid black;width:100%;">
     <tr>
         <td style="width: 50%; font-weight: bold; padding-top: 10px;">
             <div class="form-group hover">
                 @Html.LabelFor(model => model.RequestNo, htmlAttributes: new { @class = "control-label col-md-5" })
                 <div class="col-md-7" id="RequestNo">
                     @Model.RequestNo
                 </div>
             </div>
         </td>
         <td>
<div class="col-md-7" id="Reason">
                @Html.TextAreaFor(model => model.Reason, new { id = "txtreason", @class = "form-control" })
</div>
        </td>

     </tr>
</table>

action will update on database will be as below 

public JsonResult  Edit(ResignationRequester req)
{
    dynamic responseData = new ExpandoObject();
    responseData.success = false;
    responseData.message = "";
    string query;
 
        ResignationRequester workforceRequest = ResignationupdateReason((int)req.RequestNo,req.Reason);
    return Json(responseData);
}

and javascript function will make update will be 

java script function 

function changereasonbasedonrequestno() {
    console.log("fire event")
    var ResignationRequester = new Object();
    ResignationRequester.RequestNo = document.getElementById("RequestNo").innerHTML.trim();
    ResignationRequester.NoticePeriod = document.getElementById("Reason").innerHTML.trim();
}