JSOM Create a file in a document set

I want to create file in a document set with JavaScript / jQuery.

function createfile (url)   
{  
    var clientContext;  
    var oWebsite;  
    var oList;  
    var fileCreateInfo;  
    var fileContent;  

    clientContext = new SP.ClientContext.get_current();  
    oWebsite = clientContext.get_web();  
    oList = oWebsite.get_lists().getByTitle("Documents"); 

    fileCreateInfo = new SP.FileCreationInformation();  
    fileCreateInfo.set_url("my new file.txt ");  
    fileCreateInfo.set_content(new SP.Base64EncodedByteArray());  
    fileContent = url;  

    for (var i = 0; i < fileContent.length; i++)   
    {  
        fileCreateInfo.get_content().append(fileContent.charCodeAt(i));  
    }

    this.newFile = oList.get_rootFolder().get_files().add(fileCreateInfo);  
    clientContext.load(this.newFile);  
    clientContext.executeQueryAsync( Function.createDelegate(this, successHandler), Function.createDelegate(this, errorHandler));  

    function successHandler()   
    {  
       console.log("FILE CREATED!");
    }  

    function errorHandler()   
    {  
       console.log("File Creation Failed: " + arguments[1].get_message());  
    }  
}  

This will create the file in the "Documents" lists. My goal is to create the file directly in the document set which is called "MyTest".

With the SharePoint Client Browser I can see that the folder is located at: Documents / Root Folder / MyTest