How do I make a request using dojo.xhrGet() with Basic Authentication?

I'm trying to make a dojo xhrGet request using basic authentication but I keep getting 403 forbidden error. I can make the request with curl from the command line, so I know my credentials are valid. When I check the request headers, the Authorization: Basic header isn't even being set. What am I doing wrong:

    var lookupArgs = {
     url: "",
     handleAs: "text",
     load: function(data) {
         content_node.innerHTML = data;
     error: function(error) {
         content_node.innerHTML = error;

Request Header:

GET /myapi/endpoint HTTP/1.1
Connection: keep-alive
Origin: http://my-origin
X-Requested-With: XMLHttpRequest
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/65.0.3315.3 Safari/537.36
Content-Type: application/x-www-form-urlencoded
Accept: */*
Referer: http://my-origin
Accept-Encoding: gzip, deflate, br
Accept-Language: en-US,en;q=0.9