Enable passwordless sudo as a specific user

Follow these steps:

1. Edit sudoers file (this can be present in /etc/sudoers.d/file_name)

sudo visudo -f /etc/sudoers

2. Add line at the end of the file

usernameusedforlogin ALL=(ALL) NOPASSWD:ALL

3. Save file

esc :wq!

Revised answer from comments: If you place directives below #includedir, they are ignored. Move the line next to the sudo or admingroup definitions, or place it in separate file in /etc/sudoers.d/.