to use the native Docebo certificates, they must be attached to the course before the user is marked as completed. 

If you cannot do this (as you mention above), you create a SCORM object that would generate the certificate however it may be tricky to bring in the user name - will likely require some proper coding). When it’s set-up as a learning object, you can then set the prerequisites for the objects to ensure they only access the certificate object once the completed the previous objects.

You can also look to build the certificate directly in your course content.