The custom_dictionary command is available for the WebSpellChecker server/on-premise version since the release of v5.23.0.0 in July 2022. The custom dictionary's Web APIs for the Cloud version are under development and are coming soon. 

• If you are using the Cloud version, you can create a custom dictionary from your account on our website. Refer to Configuring cloud custom dictionary guide to learn how to create and manage dictionaries for the Cloud version.
• The holders of the previous Server version need to follow Company custom dictionary guide.