To add a person to "Favorites", go to "Messages" and place your cursor over the circle with 3 dots, click and select "Add to Favorites" from the dropdown. Or you can go to the desired user profile and find the yellow button "Add to Favorites" at the top right corner.
To add a person to the "Favorites", open the list of all dialogs in the mobile version, select the desired dialogue, hold and swipe it to the left.
You will have three options: star, flag and basket. Star - adding a dialog to favorites.
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