Your bot can now see and act on a private message a member sends it directly, not only what happens inside your server.
Bot receives a direct message event
A new event type in the Events tab, fires whenever a member DMs your bot. Build a workflow on it the same way you would for any other event, this is the starting point for anything you want to happen from a private message, a ModMail style handoff to your staff, a private survey, a signup flow, whatever you need.
Wait for DM Reply block
Optionally sends the member a prompt, then waits for their next direct message, with a timeout you set, writing what they typed into a variable and branching Success or Timeout. Chain several of these in a row (an embed or reply after each one) to build a full back and forth conversation entirely in DMs, for example a multi question form: ask a question, wait for the answer, ask the next one, and post everything collected to a server channel once it's done. It works whether the conversation starts from a Direct Message Received event or from a button someone clicked in your server, so you can kick a private flow off from a server command just as easily.
ModMail module
A ready made example built entirely from the pieces above. The first time a member DMs your bot, it opens them a private staff channel (optionally under a category you choose, and visible to a staff role you pick) and posts their message there. Every later DM from that member gets added to the same channel, and every reply staff types in that channel goes back to the member as a DM. A Close button on the thread's intro message removes it once the conversation is resolved.
Tip: Turn ModMail on from your Modules tab like any other turnkey module, no workflow builder needed for it, but the same Direct Message Received event and Wait for DM Reply block it's built from are yours to build something completely different with.