Steps to reproduce:

1. Add a link.
2. In the "search" text field, enter a page with a namespace. Manual:Contents on mediawiki.org in this example.
3. Click the newly created link.
4. In the popup, click the "preview" link (.ve-ui-mwInternalLinkContextItem-link).

Actual result:
Opens the URL Manual:Contents.

Expected result:
Opens the URL https://www.mediawiki.org/wiki/Manual:Contents.

Additional information:
Browser: Firefox 62.0.3 (64-bit)