You can't. You need an elliptic curve key. That is way more complex and secure that converting something like "S3curEChaNNel_01?" to a byte array.
An example for a ECC key would be an ECC certificate issued by an certificate authority.
If you don't want to go the PKI road. Create one and and import as follows:
var key = CngKey.Create(CngAlgorithm.Sha256);
var bob = new ECDiffieHellmanCng(key);
var x = bob.Key.Export(CngKeyBlobFormat.EccPrivateBlob);
bob = new ECDiffieHellmanCng(CngKey.Import(x, CngKeyBlobFormat.EccPrivateBlob))