The game does not natively support widescreen. That said, you could try the WK Universal Unofficial Patch to enable this feature:
- Download the patch and extract it.
- Copy the file "d3d8.dll" to the game data folder.
- If the game was installed via Galaxy, please select the game in Galaxy, click on More -> Manage Installation -> Show Folder
- If the game was installed via offline installer, then you will have to manually navigate to the game installation folder, by default it's: C:\GOG Games\game-title\