This information is for TaskPaper 2 which is old and outdated. TaskPaper 3 is the current version of TaskPaper. For TaskPaper 3 support please visit the support forums.
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