Thanks @AndreasHall .
I am able to open Workforce via the link however, to open the assignment it only seems to work if I hardcode the GlobalID, if I pass it as a parameter then it won't. I have tried creating the link via Arcade as well as via the popup source.
e.g.
This works:
<a href="https://workforce.arcgis.app?mapID=92a54bce9afa4de9b741f3d2e2cbed23&assignmentID=7a009267-7e91-40a4-9054-ceef26a3211f">Workforce assignment</a>
This will not:
<a href="https://workforce.arcgis.app?mapID=92a54bce9afa4de9b741f3d2e2cbed23&assignmentID={GlobalID}">Workforce assignment</a>
Checking the links in the Desktop web map they both produce the same link, so it seems very odd that when parsing a field parameter it fails.
Any further suggestions or is this a bug with Workforce that it just won't work?