You can try, as the position is equal to the center of the map :
position: map.getCenter(),
Or, i think a proper way, put prompt results into variables :
var lat = prompt("Latitude");
var lng = prompt("Longitude:");
var currentLatLng = new google.maps.LatLng(lat,lng);
map.setOptions({center:currentLatLng});
and then
position : currentLatLng,