Through trial and error, I have discovered a way to embed hyperlinks in Designer and Publisher documents.
1. Find and copy an existing hyperlink in a PDF, InDesign or similar document.
2. Paste into your Designer or Publisher document.
3. Change the text to the desired new link. This can be to a web page or an email mailto.
I also tried pasting the link beneath a different text layer that could act as an anchor, and changing the link text opacity to zero, with mixed results. (So the actual text of the link would not appear).
This works for me using the Publisher demo and Designer on an iMac running Mojave… no guarantees for other setups.
Serif link test 21.pdf