I don't know for certain, but you can probably contact firstname.lastname@example.org by e-mail and ask. There's a Knowledgebase solution that suggests that is the right process, at least for courses taken through Red Hat Online Learning: https://access.redhat.com/solutions/1123303.
Hi Dath - Looks like the above solution is a little outdated. Depending on what type of course you have completed, you should be able to download the course completion certiticate for the learning portal itself. You may refer to the below FAQs if you are a Red Hat Learning Subscription user - https://www.redhat.com/en/services/training/docs/rhls-faq
If you have a current learning enrollment and need further assistance, you can also submit a service request by following the steps here - https://access.redhat.com/role-support