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 

     

         

             

                 @Html.LabelFor(model => model.RequestNo, htmlAttributes: new { @class = "control-label col-md-5" })
                 

                     @Model.RequestNo
                 
             
         
         


                @Html.TextAreaFor(model => model.Reason, new { id = "txtreason", @class = "form-control" })

        

     

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();
}