You can enable the filter from Configuration -> Text formats and editors -> Configure -> Enable the Hel.fi: Link converter filter. This must be run after Convert URLs into links filter if enabled.
The filter parses all links from markup fields and runs them through #type => link render element, so they can be processed the same way as other links are processed. See src/Plugin/Filter/LinkConverter.php.
We override the default link #preprocess callback in helfi_api_base_element_info_alter() hook to run all our links through a template to figure out whether the link is external or not. See:
- src/Link/LinkProcessor.php
- src/Helper/ExternalUri.php
- tests/themes/link_template_test_theme/templates/helfi-link.html.twig (this is overridden in
hdbttheme as well).