Following up on this for 8.1 or Mac users…
I went down the deep-link rabbit hole but was unsuccessful and opened a support ticket with IA (177260).
If you're on 8.1 deep-linking to a specific project isn't fully implemented yet — it'll open the Designer Launcher but won't actually load the project. IA is aware and said they'll follow up when that changes.
For 8.3 users it does work, and if you're on macOS Tahoe there was a specific deep-link bug fixed in 8.3.5 (IGN-15850): Nightly 8.3 Changelogs - 2026 and beyond - #58 by system