CreateNewTag

To add a @priority tag to the selected entry:

tell application "TaskPaper"
    tell front document
        tell selected entry
            make tag with properties {name:"priority"}
        end tell
    end tell
end tell

To to update the value associated with an existing tag:

tell application "TaskPaper"
    tell front document
        tell selected entry
            set value of tag named "priority" to "1"
        end tell
    end tell
end tell