KeyAlgorithm for SHA256
KeyAlgorithm parameter defines the public key algorithm which is not related to signature algorithm (what you are trying to accomplish). Instead, you need to use -HashAlgorithm parameter and specify SHA256 as a parameter value:
New-SelfSignedCertificate -DnsName "MyCertificate", "www.contoso.com" ` -CertStoreLocation "cert:\LocalMachine\My" ` -Provider "Microsoft Strong Cryptographic Provider" ` -HashAlgorithm "SHA256"