When adding https://www.youtube.com/c/yourusername, the "c/" is automatically removed, changing the URL into https://www.youtube.com/yourusername, which might work in most cases but not in all cases

Some examples:
https://www.youtube.com/c/pierreperretofficiel works
https://www.youtube.com/pierreperretofficiel 404

https://www.youtube.com/c/VincentPeiraniofficial/ works
https://www.youtube.com/VincentPeiraniofficial/ 404

https://www.youtube.com/c/JamesBakerOn works
https://www.youtube.com/JamesBakerOn 404