After your first top up you will be asked to enter a 4 digit code into MuchBetter to verify that you are the owner of the card used to top up.
Usually you can find the 4 digit code on the bank statement of your card, it will appear after MB*, MBETTER* or MUCHBETTER*. Note that some times it is there in a confirmation SMS from your bank, if you get an SMS after every card transaction.
The 4 digit code is 4187 in this case.
Some banks may not show the code, in that case, please email firstname.lastname@example.org with a screenshot of your bank statement (showing your initial MuchBetter top up and the account holders name) and a photo of you holding your card, covering the middle 8 digits.