If you use English-US or English-UK, the back tilde (`) key is on Layer 3 in the top left key of the Dygma Defy and Dygma Raise 2.
If you want the tiled key ~, you'll need to press Shift + `.
You can also configure the ` key to anywhere you want.
- Go to the layout editor
- Select the key that you want to assign ` to
- Assign ` from the key picker below
If you want to add ~, assign ` and then add Shift under the Create Shortcut section.
This will apply Shift to the ` every time you press that key, so you'll automatically get ~.