Yes you can use whatever you want
You may want to check the SimpleChat example (yes it is from SFS1x, but you'll be able to see how easy it is to implement your own way), which uses just a simple TextField for the login. You can find it under [SFS Installation Folder]\Examples\AS2\02_simpleChat.
Basically, you just need an input Textfield and a movieclip/button. Then, when you press that button, you just call the loadConfig()
You can also check this
example, which explains how to build a simple flash application that connects to the server.
//SFS tutorials: http://sfs-tutor.blogspot.com/
- I no longer update it. Some examples may be bugged