See all the steps to send your Postmark newsletter for WordPress
In order to send with Postmark, you will need to have an Postmark account.
Log on your account and access the server configuration. If you don't have one, you should create one. Then edit the server and access the API Tokens part to generate one if needed and get it.
Use this token in the AcyMailing configuration and you can then test it thanks to the "Test credentials" button.
You can also access the Sender signature part in Postmark to configure it (allowed sender address, domain, DKIM...).
You should then use this sender in the AcyMailing configuration.