Authorize API call using OAuth access token and AJAX

[Virtamove IT]

How am I supposed to authorize an API call to create a new session with OAuth? I have gone through the steps to generate an access token, but I am always returned 401 ( "You are not authenticated. Authentication required to perform this operation." )

my call:

$.ajax ({    
url: "https://myjira.com/rest/auth/1/session",    
type: 'GET',    
beforeSend: function(xhr) {        
    xhr.setRequestHeader("Authorization", "Bearer <accesstoken>");
    xhr.withCredentials = true;    
},    
success: function(response) {        
    alert("success");    
},    
error: function(error) {        
    alert("error");    
}});

Doing this same call using basic authentication works fine, such as:

var username = $("#username").val();
var password = $("#password").val();
$.ajax ({    
url: "https://myjira.com/rest/auth/1/session",    
type: 'GET',    
beforeSend: function(xhr) {        
    var base64 = btoa(username + ':' + password);     
    xhr.setRequestHeader("Authorization", "Basic " + base64);     
    xhr.withCredentials = true; 
},    
success: function(response) {        
    alert("success");    
},    
error: function(error) {        
    alert("error");    
}});

How am I supposed to authorize this call for a new session using oauth? I've spent days now searching for a working example with no luck. Thanks!