Add a log of change in Activity stream

Have a nice day.
I added a new field for task, to flag a special purpose, by my own plugin.
Everything is good: I can hook and work with database, display on the board and task detail.
But there is no action get logged in Activity stream when I update new field.
Can anyone guide me to work add a log of change for Activity stream. ( in source code, from my plugin )
Thank you very much.