The problem with your code is that you put the headers inside the call object. Wix’s .fetch() actually works by:
.fetch(url, options)
The options is an object which looks like this:
{
method: 'get',
headers: {
'Content-Type': '<the target content type>',
'Authorization': 'Bearer <your api key>'
},
body: 'key1=value1&key2=value2'
}