On Mikrotik terminal:

[admin@MikroTik] /certificate> create-certificate-request 

Fill all data. Country, state, city, mail, domain, organisation…etc.

It will create 2 files:

certificate-request.pem

private-key.pem

On linux box:

openssl rsa -in private-key.pem -text > certificate-request2.pem
openssl x509 -req -days 9999 -in certificate-request.pem -signkey certificate-request2.pem -out mikrotik_ssl.crt

Then back on Mikrotik:

/certificate import file-name=mikrotik_ssl.crt
/certificate import file-name=certificate-request2.pem
/ip service set www-ssl certificate=cert1