Hi sid,
Thank you so much for your response 
Thanks to your answer I was confident that this was indeed the right approach. I just tried the whole setup over again (seemingly the same as my previous attempt). However, this time it worked! 
What doesn't work is actually saving the API key. But I already replied to this older thread:
So hopefully this will get fixed soon, now my maps have this ugly 'for development purposes only' watermark.