How do I do it with the BGMPLAY command?

Almost all legacy game platform had simple and cheap sound generator IC. If you reproduce such 8-BIT sound, you should use @144-@150 or @151 for music instrument, and use less than three music parts.

Do you know how to write MML?

Yes, thank you.

Additional information. There are some hidden instruments in SmileBASIC. For example, @256 is saw tooth wave, one of the most famous 8-BIT sound. And it is better to use @266 as noise source rather than @151 to reproduce like wind blowing sound.