This was my issue too. I assumed that the Client ID and Client Secret were required (which I believe is the case for controlling Spotify which I don't need). Really disappointed with the support though. I posted asking for help and the dev just replied, 'Can't replicate the issue, it's a problem on your end'. When I asked how to fix it, they again just replied 'I don't know'. I might be old school, but when people ask me for help on something I make, I try to answer their questions and help them until they're satisfied, especially when it's public simply because 1. makes you look better to everyone else that you genuinely try to help and 2. just maybe answering the question will answer the same question for many others so they don't need to even ask.
Regardless, I worked it out myself in the end, but if the dev tried to help a little more, they may have been able to help me much quicker.