Importing from iMessage
CustodyComms imports an iMazing .csv export of your iPhone messages, the same way it imports a WhatsApp .txt chat export. Apple does not include a way to export iMessage conversations, so use a desktop tool that reads an iPhone backup, such as iMazing (Mac and Windows), and export the conversation as a CSV file. CustodyComms does not import iMazing's PDF or TXT exports.
In the desktop app, click "Import Messages" in the left sidebar. Under "Where are your messages from?", click "Apple iMessage", then click "Choose File" and pick the .csv file. A group chat that iMazing labels with a group name, instead of a person, may not be imported. One-to-one conversations in the same file import normally.