CreateNewEntry

To create a new task at the end of document:

tell application "TaskPaper"
    tell front document
        make new entry with properties {name:"- my new task"}
    end tell
end tell

To create a new task in an existing project:

tell application "TaskPaper"
    tell front document
        tell project named "Inbox"
            make new entry with properties {name:"- my new task"}
        end tell
    end tell
end tell