https://docs.netgate.com/pfsense/en/latest/vpn/openvpn/configuring-a-site-to-site-pki-ssl-openvpn-instance.html
https://docs.netgate.com/pfsense/en/latest/book/openvpn/site-to-site-example-configuration-ssl-tls.html
https://www.netgate.com/resources/videos/site-to-site-vpns-on-pfsense.html
And you need a Port Forward on OpenVPN Server side B on Mikrotik (OpenVPN server Port to pfSense).
-Rico