Threema sends messages via the so-called Threema gateway. This is a service provided by Threema for a fee. The gateway provides you with the required sender ID (gateway ID) and the API key (secret).
More information about the pricing model and setting up the gateway can be found here:
Threema Gateway – Send and receive Threema messages with your own software.
To receive messages, either a normal Threema account or a Threema Work account is required. For use in a company, the use of Threema Work is recommended. This allows Threema accounts to be created and managed for a small monthly fee.
Details of the pricing model, as well as further information, can be found at the following website:
Threema Work – The Privacy-Compliant Enterprise Messenger – Threema Work

Please note:
Sending each message costs gateway credits! DataSuite only forwards the requests to send the message to Threema Gateway. Make sure that there are always enough credits available.
To set up the account in DataSuite, select the Accounts menu item on the left and click on Create new account in the top right-hand corner.
In the dialogue, select the option: Threema -Account and click on Create.