Most extensions, such as .com, a domain transfer takes place via an 'Authorisation code', also called a transfer code or token. However, some extensions have a different procedure for moving a domain. A (partially) separate procedure applies to the .amsterdam extension.
The first steps for transferring a .amsterdam domain are the same as for a .com domain. You can view these steps in the tutorial for 'Transferring your domain name to TransIP'.
After finalising your order, you will see a notice when visiting the domain in your control panel. You can do so by going to 'Domain & Hosting' and clicking your domain name. The notice states that an email was sent to the email address of the administrative contact for the domain. This is the administrative email contact at the current provider.
This is the only difference from tranferring a .com domain name. After confirming the transfer via the email mentioned above, the transfer will be processed by the registry. This can take up to a maximum of 7 days.