If it says the API key is listed as a header, more than likely you need to set it in the headers option of your http request. Normally something like this :
headers: {'Authorization': '[your API key]'}
Here is an example from another Question
$http({method: 'GET', url: '[the-target-url]', headers: {
'Authorization': '[your-api-key]'}
});
Edit : Just saw you wanted to store the response in a variable. In this case I would probably just use AJAX. Something like this :
$.ajax({
type : "GET",
url : "[the-target-url]",
beforeSend: function(xhr){xhr.setRequestHeader('Authorization', '[your-api-key]');},
success : function(result) {
//set your variable to the result
},
error : function(result) {
//handle the error
}
});
I got this from this question and I'm at work so I can't test it at the moment but looks solid
Edit 2: Pretty sure you should be able to use this line :
headers: {'Authorization': '[your API key]'},
instead of the beforeSend line in the first edit. This may be simpler for you