Exchange 2013, how do i assign a certificate to the SMTP site?

Event ID 36871: A Fatal Error Occurred While Creating An SSL (client or server) Credential
This behavior is caused by the SMTP service processing an incoming EHLO command if no certificate is assigned to an SMTP site. This message is logged twice, once when the SMTP service starts, and once when the first EHLO command is received.
Simple Mail Transfer Protocol (SMTP) controls how email is transported and then delivered across the Internet to the destination server. The SMTP EHLO command enables the server to identify its support for Extended Simple Mail Transfer Protocol (ESMTP) commands.

This is an erroneous Event log entry. You can safely ignore this message. To prevent this Event log entry, you must assign a certificate to the SMTP site.

Great.  I have that exact same info.  I'm asking HOW to assign a certificate to the SMTP site?  :)  I don't want to ignore the error.  I want to fix the error.

In the Exchange Administration Center navigate to Servers -> Certificates and choose the server that has the SSL certificate you wish to assign. The certificate must already been in a valid status before you can proceed further.

Maybe restart server will be required

Step by step you can find here

https://practical365.com/exchange-server/exchange-2013-assign-ssl-certificate-to-services/

My two cas servers have certs assigned. Do I need to assign the same cert to the two mail box servers?

Your servers are talking to each other ? How you email flow looks like ?

Email is flowing fine. No mail flow problems, just wanna get rid of the error.