Hi @Mạch_Quang_Luân , welcome to the forum 
Seeing that you are using tags instead of custom fields and there is no ‘+’ icon near your red arrow, I’m assuming that you are on the free plan…?
On the Premium & Business plans you can add custom fields to your My Tasks, in which case you could create a text field called ‘Personal Notes’ and type in the notes you mention above.
These notes would remain private just to you (unless you share your My Tasks with someone else)
You can also refer to the topic here: Make way for Custom Fields in My Tasks!