views.py

def some_view (request):
    # Could substitute `None` with some other default value
    key = request.POST.get("key", None)
    return render(request, "mytemplate.html", {"key": key})
    
# In mytemplate.html you'll now have `key` available with whatever was submitted via the form. Note
# that there is NO validation here, you should probably be using a form to capture the input, 
# validate it, and return it. https://docs.djangoproject.com/en/dev/ref/forms/api/