Index.cshtml.cs
public async Task<IActionResult> OnGet(string qstring)
{
Console.writeline(qstring)
return Page();
}
Index.cshtml
@page "{qstring?}"
<form method="get" asp-page="Index">
<select name="qstring">
<option value="one">one</option>
<option value="two">two</option>
</select>
</form>