First navigate to https://helm.shiptheory.com/citipost/edit/
Here you can enter your API Username and password.
If you don't have these or are unsure as to what they are. 13ten will provide you with them, please call 13ten on 0800 783 1310 to speak with them.
Once you've entered the API username and password, hit test connection to confirm they are correct, then hit save.