duration_ms in the allocation so the app can show the length. Then call POST /v1/chats/{chatId}/voicememo (reference) with the completed attachment_id. The SDK method is relay.chats.sendVoicememo.
Relay refuses anything that is not audio/* on this route. Instead of an upload, voice_memo_url accepts a public HTTPS audio URL up to 10 MiB (10,485,760 bytes), checked the same way as other URL media. Send one of the two fields, not both.
Send the voice memo
Send the body to the voice memo route:202 with the message and a voice_memo object holding the Attachment ID, a download URL, and the file’s metadata. The person hears it as a voice memo on their phone rather than as a file.