How to find postfix public domainkey in plesk server for dkim dns settings?

After plesk dkim activation any services (like opendkim) not required.
The keys store in «/etc/domainkeys/», but public dkim keys do not store.

We can easily generate them:

# openssl rsa -in /etc/domainkeys/somedomain.tld/default -pubout -out /etc/domainkeys/somedomain.tld/public
# writing RSA key


After that you will see:

# cat /etc/domainkeys/somedomain.tld/public
-----BEGIN PUBLIC KEY-----
MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDNP/0esApEWVmQEtjbT8WGJSyPhKpNvxuxHDSf/TChc1ssgO+DWtXOdFyd/jM1L+vtBUOXLewmrWJ+shnAiZOibyR2LtCvmBZ44K08UvC6Xqnz8Jh9BLv9K0KZ/3tX7UU0DMHQ/zWb1Ya6cWHfkfJc4H+DkqAGQhQPZMk3nrwXVwIDAQAB
-----END PUBLIC KEY-----

And than you can set up it in you dns hosting provider, like:
name: default._domainkey
type: TXT
value: "k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDNP/0esApEWVmQEtjbT8WGJSyPhKpNvxuxHDSf/TChc1ssgO+DWtXOdFyd/jM1L+vtBUOXLewmrWJ+shnAiZOibyR2LtCvmBZ44K08UvC6Xqnz8Jh9BLv9K0KZ/3tX7UU0DMHQ/zWb1Ya6cWHfkfJc4H+DkqAGQhQPZMk3nrwXVwIDAQAB"

For dkim testing you can use this service.