If your intention is to put a background music in the stories of Facebook, start the app of the famous social network, go to the News section of your account and press the + button at the top left to create a new story. Alternatively, tap the ☰ button, press on your name to access your Facebook profile, and tap the Add a story option.
Now, on the Create a Story screen, select the Music option and press the Add button for one of the songs visible in the Popular section. If, on the other hand, you want to search for another song, enter the title in the Search music field, tap the song that interests you (so you can listen to a preview) and press the Add item, to add it to your story.
After that, select the part of the song you prefer (you can only play 15 seconds) and press the Next button, then tap the sticker for the chosen song, to customize the style, or drag it down to delete it.
If, on the other hand, you want to create a story with a video made on the spot, tap on the Camera item, select the Music tab and choose the song that interests you. After that, press the ▶ ︎ button, to record the video, and you’re done.
Regardless of the choice made previously, choose whether to add other elements to the story, by clicking on one of the available options: Effects, to apply an effect to the story (Gradient, Vintage, Pencil, Engraving, etc.); Background (the round icon) to choose the color or image to use as the background of the story; Sticker (the icon of the smiling face), to add stickers, GIFs, polls and much more and Text (the word Aa), to insert a text to the story you are creating.
At this point, tap on the Privacy option, to set the visibility of your story, and press the Share button in the story, to publish it. To learn more about it, my guide on how Facebook stories work can help.