Using openssl to generate private Key

[nicolas prochazka]

Hello,
I'm trying to use openssl to generate curve key ,
as : openssl genpkey -algorithm X25519
however, result seems to be not understand by wg pubkey
 openssl genpkey -algorithm X25519
 echo -n 'MC4CAQAwBQYDK2VuBCIEIFBA+aTU9C5BrNPaJe0IVBZfZF39+8WqiYET0XEEM7Ba'
| wg pubkey
wg: Trailing characters found after key

How Can I do ?
We  need to create private key with an internal tool based on openssl
library , sorry.

Regards,
Nicolas