[FEATURE] I'd like to trigger a workflow manually, with event data entered via form

From a conversation in public Slack:

"I'm wondering if it's possible to set up a trigger that accepts some input fields in the UI.

That would prevent me from having to activate a workflow by typing that info on the website instead of creating sending an HTTP request via curl in the command line.

The use case I have in mind is administrative tasks. For example, when a user sends me an email saying they want to cancel, I'd like a quick way to trigger an unsubscribe workflow based on their email address."