add button to detail page maps

Hi,
I need to be able to add a default value to directions input field via a button.

I’m building a site where each user would start in the same place (directions to each post/listing would be from the same start point). Thus id like to add a button that fills the directions input with a specific address and submits the form on the details page map tab.

Can it be done?

Where is the code located?

Are there any hooks/filters i should use?

Any example code already?

If its possible you could also use html 5 to submit the current users lat/lon..
Thanks,
UF