You haven’t stated the platform you’re on or what your requirements are, so it’s not really clear what you want.
Still, I’ve made an example bash script that opens surf after launching syncthing. Syncthing is launched in a daemonized
screen program and surf is launched in the current shell. Note that when the shell exits, surf is also killed. If you don’t want that behavior you need to specify what your intended behavior is. If you’re already starting syncthing via systemd or something, you should instead modify your systemd script.
screen -dmS syncthing syncthing -no-browser
sleep 5 # Wait for GUI to come online
screen is a program that you may need to install on your system first. It has the nice benefit that it can easily daemonize stuff while also preserving the ability to read/write to stdin/stdout of the process, by attaching or detaching from the screen.
Of course there are many more possibilites, which mostly depends on how you’re starting syncthing. Anway, the general concept is: Launch syncthing, then open your browser.