Custom Link – core/navigation-link
Add a page, link, or another item to your navigation.
- Custom Link : Post Link – core/navigation-link post
A link to a post. - Custom Link : Page Link – core/navigation-link page
A link to a page. - Custom Link : Category Link – core/navigation-link category
A link to a category. - Custom Link : Tag Link – core/navigation-link tag
A link to a tag.
Use the Custom link variation of the Navigation link to create a custom link.
Screenshot

Example
There should be two custom links: Custom link 1 and Custom link 2.
Notes
- The Custom Link is expected to be used in the Navigation – core/navigation block.
- You can programmatically create a Custom link.
- You can’t explicitely add a Custom link block into a post/page or even Navigation block.
- They’re created automatically, so it seems.
HTML for a custom link
<!-- wp:navigation-link {"label":"Custom link 1","url":"Custom%20link%201","kind":"custom","isTopLevelLink":true} /-->