✈ Get a YouTube video that is the character's theme song. (Official videos are better, just to avoid any issues later in time of the video not working for whatever reason.)

✈ Click on "Share" option for the video and then "Embed" option when the modal pops up. The embed code should look something like this without spaces next to the < and >:

< iframe width="560" height="315" src="https://www.youtube.com/embed/VBmMU_iwe6U"
frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen >< /iframe >

✈ From the embed code, copy what is inside the quotations of [ src = "" ] (bolded for your visual ease), which should be a youtube link. (I'll refer to this as the theme song link now.)

✈ Go back to your app and click on "Source" on the top left. Control+F or Command+F, and look for "iframe" within the code. This code should look like this without spaces next to the < and >:

< div style="width:180px;height:100px;float:left;margin-top:10px;" >< iframe frameborder="0" height="100"
src="https://www.youtube.com/embed/ip7kR2SAdGs"
width="180" >< /iframe >< /div >

✈ Replace what is in the quotations of [ src = "" ] in the iframe - which I bolded for your visual ease - with the theme song link.

✈ Click "Source" on the top left of the app again. Once you save and look at the app again, the video should be replaced. Ta-da!