This is only possible as ADMIN.
Can I create a new admin in Bash? For TVheadend?
ADMIN user (and password) it's a security thing, doesn't exist an easy way to bypas it. The only time when you able to create an admin user, is the first time accesing the web interface. After that only an admin user have right to create another user...
The "easy" (but long) way is to reset the TVheadend server addon, create again the admin user and make all the settings again...
OR... you can play a bit to make a backup of the TVheadend server addon data (".kodi\userdata\addon_data\service.tvheadend43" - copy it somewhere), reset/reinstall the addon, make admin user in the web interface, stop the addon (ex. addon settings - disable addon), restore everything from addon data backup excepting the "passwd" folder, reboot, enable... and maybe...