(1) In the board settings > Others, 'Enable Announcement' to 'Yes'.

https://www.learnus.org/pluginfile.php/4632352/mod_ubboard/article/799648/1-1.png

(2) When writing on the board, check the 'Notices' at the bottom of the title.

https://www.learnus.org/pluginfile.php/4632352/mod_ubboard/article/799648/2-1.png

Posts checked with 'announcement' will be located at the top of the board.

https://www.learnus.org/pluginfile.php/4632352/mod_ubboard/article/799648/3-1.png