You can access form elements via their "name" property using Request.Form
string val = Request.Form["GuestsRooms"]
Note you'll get the value of your dropdown, not the text, so you'll see "3-1" or something like that. If you want both then amend your options
option value="3-1|3 adults in 1 room">3 adults in 1 room</option>
then on the server do a string.Split on the | to separate the value and the text. Google "c# string.Split" if you don't know how to use it.