Using the Zcash memo field
In Zcash, memos are encrypted messages that accompany shielded transactions.
With Zodl, transactions to other shielded addresses can include a memo of your choosing, which only the recipient can view.
The memo can contain up to 512 characters, containing any information you deem relevant, whether that's the purpose of the transaction, words of encouragement, confessions, or wedding vows (as has been the case in Zcash).

Recipients can see the memo when tapping on the incoming transaction's details in their Activity panel.
Memos cannot be sent or received when the recipient is a transparent address.