To open a Kontist account we need your 11-digit personal tax identification number (not your VAT number nor the tax number on your VAT return).

More information here:

https://intercom.help/kontist/en/articles/1559727-where-do-i-find-my-11-digit-personal-tax-id-number

Enter your tax ID number in the Kontist App like this: Go to menu > settings > personal data > tax ID