How to pass the parameter into @Url.Action

Currently, I want to pass the value from text box into Url.Action but I don't know how to do.

Create View

@using (Html.BeginForm())
{
   @Html.AntiForgeryToken()

   <div class="form-horizontal">
       <hr />
       @Html.ValidationSummary(true, "", new { @class = "text-danger" })


       <legend style="border-bottom: 3px solid #e5e5e5; text-align: center">TẠO BÁO CÁO TỒN KHO</legend>
       <div class="container-fluid">
           <div class="row">
               <div class="row ">
                   <div class="form-group">
                       <label>Chọn </label>

                       @Html.EditorFor(model => model.Thang, new { htmlAttributes = new { @class = "form-control", style = "width:80px" } })
                       @Html.ValidationMessageFor(model => model.Thang, "", new { @class = "text-danger" })
                   </div>
               </div>


               <div class="row">
                   <div class="form-group">
                       <label>Chọn năm</label>
                       @Html.EditorFor(model => model.Nam, new { htmlAttributes = new { @class = "form-control", style = "width:80px" } })
                       @Html.ValidationMessageFor(model => model.Nam, "", new { @class = "text-danger" })
                   </div>
               </div>



           </div>
       </div>
       <div class="form-group">
           <input type="submit" style="margin-left:15px" class="" value="" />
       </div>
      
       <div class="form-group">
           <a href='@Url.Action("Index", "",new { thang = 1, nam = 2020 })'>
               <input type="button" class="fa fa-pencil-square" /> 
           </a>
       </div>
 
   </div>
}

I want to pass Url.Action with value which I typed from Model.Thang and Model.Nam instead of static value (1,2020). But when I change

 <a href='@Url.Action("Index", "",new { thang = 1, nam = 2020 })'>
                <input type="button" class="fa fa-pencil-square" /> 
 </a>

into static value (1,2020). But when I change

 <a href='@Url.Action("Index", "",new { thang = Model.Thang, nam = Model.Nam })'>
                <input type="button" class="fa fa-pencil-square" /> 
 </a>

It appeared errors.

System.NullReferenceException H Result = 0x80004003 Message=Object reference not set to an instance of an object.

Thank you for your helping!