In most cases, you can simply do this:
moment.tz("2013-08-26 16:55:00", "America/Los_Angeles")
If you require input other than ISO8601, then specify the format string as the second parameter, and the time zone as the third:
moment.tz("8/26/2013 4:55 pm", "M/D/YYYY h:mm a", "America/Los_Angeles")
And if you need to use moment’s “strict parsing” mode, then that goes in the third parameter, and the time zone moves to the fourth position:
moment.tz("8/26/2013 4:55 pm", "M/D/YYYY h:mm a", true, "America/Los_Angeles")