To link to Ulysses sheets or entire groups from other apps, such as Obsidian, use Ulysses’ Callback URL.
On Mac, press and hold the Option key (⌥), right-click a sheet or group in the sheet list, and choose Copy Callback URL. You can also use the keyboard shortcut ⌃⌥⌘C (Control–Option–Command–C) to copy the link. On iOS, touch and hold the sheet or group in the sheet list, select Share…, then tap Copy Callback URL. Finally, paste the copied link into a document in another app.
Please note that linking via Callback URL is only available for sheets and groups in the Notes and Projects sections of Ulysses.