Click the extension icon (you may have to pin it first) and select “Sync Steam wishlist”.
Input your numerical profile ID, and unique text profile name or simply paste them into a Steam Community profile link (e.g. https://steamcommunity.com/profiles/76561198004836934).
Here is a video tutorial how to sync your Steam wishlist with Kinguin Extensions:
Note: Make sure your Steam Wishlist is set to public for the extension to have access.