I haven’t found any tutorials or texts online for adding support for reactions on posts in MUC for ejabberd. Even documentation didn’t help so after following some leads and XEPs I finally found how to do that and it was so simple I got upset it wasn’t mentioned anywhere I could find online.
All you need to do is add this line to ejabberd.yml file:
mod_muc_occupantid: {}
That is it. Hopefully someone finds this topic when they are searching for this. From the module name you would never guess it is used for something like that.