First in the coding editor, at the top, you will see 3 tabs. ‘ Code ‘, ‘Costume’, and ‘Sound ‘. Click on the sound tab. When you have clicked on the sound tab, you will see at the bottom left there is an audio icon. If you are on PC ( computer ) hover over it with your cursor, if you are on mobile press it. A selection of icons will appear. Press the Recording button or the second closest to the audio icon When you click on that a small page will open up with a red button, but before you click it, on a different device open up and music or audio you want to input into the video or code. Now press the red recording button. And once you do start the music. When you stop the music and recording you can edit it to your liking. If you want the code I have, then press look inside to see my long code lol
What I used to make my oc is a app called ‘ Pastel Friends ‘ and edited it for the thumbnail with Ibis Paint X