How can I force custom post type to use a specific template?

If your newsletters are a custom post type you can create a template dedicated for it using the template hierarchy.

Template for a single page would be..

single-{custom-post-type-name}.php

Template for an archive page would be..

archive-{custom-post-type-name}.php

These templates get automatically selected you don’t need to add them to a page template attribute in the editor.

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