I want to get select value in HTML and use in python Flask. Because I could not find any way to get select tag option value via Flask, I get the value with JavaScript and then span it in a header. Now, I want to request the header but I could not. I get Key Error. Here is my HTML code.

        <label for="filtre_input">Neye göre karşılaştırmak istersiniz?</label>
        <select name="filtre_input" id="filt" onchange="filtreinputla();">
         <option selected="selected">Kategori</option>
         <option value="il">İl</option>
         <option value="tur">Tür</option>
         <option value="renk">Renk</option>
         <option value="duygu">Duygu</option>
       </select>

      <script type="text/javascript">
        function filtreinputla() {
        var filtreselect = document.getElementById("filt").value;
        console.log(filtreselect);
        document.getElementById("filtreselectid").innerHTML = filtreselect;
        }
      </script>
 <h3 id = "filtreselect_p" type = "hidden" name="filtreselect_p"><span id="filtreselectid"></span></h3>

Here is my app.py code.

@app.route("/nd", methods=['POST', 'GET'])
def func():
  flash("")
  filter = request.headers["filtreselect_p"]
  if filter == None:
      flash("Please, select something.")
      return render_template("index.html")
  else:
      result = my_function(filter)
      flash(result)
      return render_template("index.html")

I want to request the header or find a new approach to get select option tag value. Please, help if you can :)