You can configure and edit the payload (i.e. the message content) from the Content tab of the Cloud app studio. The rendered preview of the template will appear on the left hand side of the screen. It will be rendered for the selected preview user.
If you are using a pre-built Cloud app, the quick start template already has a message schema configured. Schema for quick start templates is read-only. If you clone a quick start template, you can edit the schema.
If you are using a cloud app template for a Custom app, you must first configure the message schema and create a message form that you can then fill.
You can use liquid variables to personalize the messages for your customers and you can also include recommendations.
For Cloud app templates, you must manually add the required tracking parameters in the template.