Using a flash outdoors requires you to factor in more variables to get a good shot. You can’t light the whole landscape with a flash and you can’t control any natural light sources, so you need to pay attention to what you can control.
In this video from the Koldunov Brothers, we learn some of basics of outdoor flash photography. First of all, when lighting your subject in a dark scene the shutter speed becomes less important; the flash is the primary light source and occurs in just an instant, regardless of the exposure time.
And though you loose that controlling factor, using a speedlight or other external flash lets you control the strength of the flash output.
The brothers go on to explore the different variables that affect the shot. Since the flash can’t illuminate the entire backdrop, shutter speed does matter if you specifically want to brighten or dim the landscape. But if you want your subject to be brighter, then you can increase the flash output (and you can also change the aperture and ISO, of course).
As with all photography, some experimentation when you’re just getting started with an external flash will help you get a sense of how to get the shot you want.
Outdoor Flash Photography. Episode 1: The basic settings. [Koldunov Brothers]