Can an outside app or lead source trigger a workflow with a webhook?

Yes. Use the Webhook trigger. It gives you a webhook URL, and whenever the outside app posts to that URL, the workflow runs. For example, a lead marketplace can post each new lead to it and the workflow can place an outbound call, send an SMS or send an email. If you can have QuickBooks send a webhook when an invoice is fully paid, the workflow can email the customer.

If the other system cannot send a webhook, set up a workflow that runs on a schedule, for example every day at 9 AM, to check instead, or ask the AI assistant for the best way to connect that platform. You can build as many workflows as you want, and you can ask the AI assistant to build them for you.