Reactions are available for posts as well as comments and chat messages.
Press and hold the Like button on posts. The selection of Reactions will then open.
Reactions are also displayed on messages or comments by pressing and holding on the message or comment.
To react to a post, you hold the cursor over the Like button for a second to display the Reactions.
For messages and comments, a reaction button is displayed on the right as soon as you move the cursor over the message or comment.