Complete a CSR request from Exchange Admin Center

Open the EAC and navigate to Servers > Certificates. and click on Pending request  and click on complete.
When you click on complete you get a popup to enter the certificate path. Please enter the certificate path. I saved the certificate in server in server, so I used localhost.
Click on OK. Your certificate is imported.
By default POP,IMAP will be enabled in certificate.

You can assign and enable service certificates in Exchange 2016/2019 from the EAC. Below are the steps with screenshots showing how to enable and assign services:

• Click on the new certificate and click the "Edit" button. 
• Check the expiry date of certificate.

• Make sure domain names and expiry date is correct. Click on Services 

• Select SMTP and IIS.  If you're also using POP and IMAP, select them as well.

The same can be done from EMS.

Run the following command to check what services have been enabled.

Get-ExchangeCertificate | fl Issuer,CertificateDomains,notafter,thumbprint,services

 
Use the following command to enable other services. You can change the services according to your requirement. but IIS is mandatory.

Enable-ExchangeCertificate -Services POP,IMAP, IIS, SMTP -thumbprint FACE6A655A03CD482AEC4AA019DC920485DC166

 
After services enabled
 
You can read this TechNet article for more information on how to assign services to certificates in Exchange 2016/2019.

Open your OWA and make sure there are no errors. Also make sure certificate shows the expiry date as expected.
 
 Now you are done. :))