document.addEventListener('DOMContentLoaded',()=>{
document.querySelector('#mainoption').onchange=()=>{
  const id=document.querySelector('#mainoption').value;
  const request=new XMLHttpRequest;

  request.open('GET','submain');
  request.send(id);
  alert("selected  "+id);
}

def submain(request):
subid = request.GET.get('id')

print(subid)

from django.http import HttpResponse

def submain(request):
    subid = request.GET.get('id')
    return HttpResponse(subid)