Not sure which plugin is that. Is it a WP plugin?
WP has a built-in “spoiler” feature triggered when adding the More tag to a post. Maybe try that instead?
I found instructions but didn’t understand how to embed on your plugin https://github.com/romanvm/TinyMCE4.spoiler
The above link is to a third-party TinyMCE plugin. If you want to use it, it will have to be made into a WP plugin.
Similar functionality is built-in WordPress too, check the documentation about the “Read More” tag.
