First go to the Web UI and select "Message Templates" under the "Appearances" section:
To create a new message click "+ Template" on the top right hand side of the page:
From here you can create a new message through the use of HTML. Door Tablet will also alert you to which tokens are needed for the message, as well as alerting you to what text is needed.
If you decide not to use any template, Door Tablet will use the developer template as standard. Note: there are a number of template options that can be used as well as alerting you to what text is needed.