You want to use the TWCore's "cascade" mechanism (see
https://tiddlywiki.com/#Cascades)
* Create a tiddler (e.g., "$:/config/ViewTemplateBodyFilters/x-fountain"), containing:
[type[text/x-fountain]then[$:/core/ui/ViewTemplate/body/rendered-plain-text]]
* Tag it with `$:/tags/ViewTemplateBodyFilter`
* Add a field named `list-before` with no field value
Notes:
* Your "cascade" tiddler can have any name. The name suggested above is consistent with the existing TWCore "ViewTemplateBodyFilters" shadow definitions.
* The tiddler content is evaluated by the TWCore cascade mechanism, and basically says: "If the tiddler's type field is equal to "text/x-fountain", then display the tiddlers body using the "$:/core/ui/ViewTemplate/body/rendered-plain-text" template
* The `$:/tags/ViewTemplateBodyFilter` tag adds your definition to the default ViewTemplateBodyFilters cascade
* The `list-before` field ensures that your custom cascade is evaluated first when the cascade is processed by the TWCore
enjoy,
-e
P.S. You might want to visit
https://talk.tiddlywiki.org/, which is a "Discourse" server that is now the TiddlyWiki community's primary discussion/Q+A area.