First, switch to the online search pane - from there you can search for a song on one of the supported sites:
Click on the search result that you want to use to preview it. If you are happy with the song as a starting point, click "Create a Chordle song from this":
You can tweak the song content before saving it. After saving, the song is added to your song library, and you will be able to find it there going forwards.