Netplan equivalent of bridged network

[weactivist]

Sorry beforehand but I can't figure out what the netplan equivalent of this is. Any help would be appreciated.

auto br0iface br0 inet dhcp
  bridge_ports DEVICENAME tap0

auto tap0
iface tap0 inet dhcp
   pre-up tunctl -u MYUSERNAME -t tap0

[TheFu]

tap interfaces are really slow. Perhaps you'd rather use a tun or bridge interface?
Sorry, I can't directly help with the 18.04 aspect of the question. netplan.io is the official reference for that subsystem.
https://netplan.io/examples has examples.

https://ubuntu.com/blog/ubuntu-bionic-netplan has a developer view.

pre-up stuff has been moved into the systemd target-wants and depends system. It isn't available in the network subsystem anymore. Every startup service has dependencies specified in their config file. The goal of this is so Y won't be started until X is started and running. A quick analysis of those dependencies by systemd allows parallel startup for the required reverse-tree of services to get to the point that "networking" is up, before "multiuser" is up. Fairly simple text files are used.

Take a look at /lib/systemd/system/networking.service file. See the After, Before, WantedBy entries? Any reference to a filename with .target in the name is a config file. Follow a few to see how they fit together.