Synonyms are hard to set up and do not always work really well. Here is how you set them up in any case:
Use → to see the synonyms tool.
Select first word (A) you want to make synonyms, then click on the second word (B)
The button in the synonym tool changes it's text to A and B are synonyms
The words will now be shown as connected in the synonym tool and can be recognized as synonyms in the practice.