Add an editor region in the template template:

<system-data-structure>
     <editor>This is my first page.</editor>
</system-data-structure>

Create a format using Velocity:

#* this is a multiline comment in Cascade
$currentPageSiteName
$currentPagePath
$currentPage
$currentPage.assetId
$currentPage.assetType
$currentPage.label
$currentPage.metadata.title*#
## This is a single-line comment in Cascade
$currentPage.getStructuredDataNode("editor").textValue
##$_PropertyTool.outputProperties($currentPage)

This is the transformation result:

This is my first page.