There are two ways to add comments:

1. When adding friends,

Mobile phone: At the end of the interface, you will be prompted to add comments.

Computer: Add friends interface will display remarks column, just fill it in.

2. Add as a friend.

Mobile phone: click on your friend's personal information. Click on the three horizontal lines in the upper right corner, and a menu bar will appear with the option of adding comments. Click on it to add comments.

Computer: Select friends → right click → add comments.

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