If you're looking to use longitude and latitude in the web address instead of the address, take a look at:
https://support.google.com/maps/answer/18539?hl=en
If you're trying to get longitude and latitude from the web viewer, it looks like it's in the url after "Click to see this area on Google Maps" in the source code. Sample below:
<a title="Click to see this area on Google Maps" href="https://maps.google.com/maps?source=embed&q=87+E+42nd+St,+New+York,+NY,+10017&iwloc=0&ll=40.756896,-73.977449&spn=0.014011,0.027466&z=16&vpsrc=6&oi=map_misc&ct=api_logo" target="_blank">
The latitude and longitude are: 40.756896,-73.977449