If you click "View E-receipt" under the reservation, you should see the certificate number displayed near the bottom under "Ticket Purchases".

For example, it'll say something like:

Penalty: NONREF*CHGFEEPLUS*FAREDIFF CXLBYFLTTIMEORNOVALUE ECERT 101234567890123456

where the 101234567890123456 is whatever the eCert number is.