How do I get license to add music to a Creative Map?

Epic Games does not grant licensing or author rights to songs. Our UEFN Content may include music that you have uploaded or created through your use of UEFN tools. However, you must fully own that music, or you must have obtained the necessary licenses to upload, synchronize and otherwise use the sound recording and/or musical work in UEFN.

For more details, please visit: Unreal Editor for Fortnite Supplemental Terms to the EULA (Opens in a new tab)
Note: Section 6.1.2 covers this info  

If you would like to add music to your Creative Map, we have some songs in-game. You can get a complete overview of audio tools in Fortnite Creative and follow the steps in this Epic Course: Fortnite Creative Course (Opens in a new tab) 
Note: Section 06 covers this info

For more information, refer to Audio in Unreal Editor For Fortnite (Opens in a new tab).