Make it a sudo user before changing default username:
*** Add user
#+BEGIN_SRC shell
sudo adduser <username>
#+END_SRC
*** Make it a sudo user
#+BEGIN_SRC shell
sudo usermod -a -G sudo <username>
#+END_SRC
*** Change default user
Change the default user which is used when starting the WSL.
Open cmd.exe and run:
#+BEGIN_SRC shell
ubuntu config --default-user <username>
#+END_SRC
Restart WSL.
Make it a sudo user before changing default username: