If you just want to send the text, just send its content:
client.on("message", msg => {
if (msg.channel.type == "dm") mychannel.send(msg.content); //mychannel is your TextChannel object
});
If you want to make that you can see the author & stuff like that you could use an embed (see how to build & send one here).
If you want to make like it's almost identical to someone's message, you could use a webhook:
guild.fetchWebhooks().then(webhooks => {
let myhook = webhooks.find("placeholder");
client.on("message", msg => {
if (msg.channel.type == "dm") myhook.send(msg.content, {
username: msg.author.username,
avatarURL: msg.author.avatarURL,
});
});
});
Hope this helped, let me know if you have any other question