In the current azure portal the deployment credentials can be set by going to App Services / select relevant item / in the APP DEPLOYMENT section / Deployment credentials / enter new credentials and save.
In the previous generation azure portal the deployment credentials can be set up by going to WEB APPS / select relevant item / DASHBOARD / Reset your deployment credentials.
Make sure you're specifying siteName\userName as your login name. The site name portion is required!
The default directory for the web content is \site\wwwroot which can be set as the initial directory for many FTP clients.