This feature is quite complicated to explain both in terms of usage (but what's the point???) and in terms of how to do it.
So, as tradition dictates, we made a great video 👇👇 (it's in French for now, but I'm sure you will get at least the sense of it)
For the experts who don't have the time, as youth say, here is the step-by-step quick guide.
1) Go to the workflow you want to be able to activate manually
2) Click on Define the fields to be filled in manually. Here you will see all the fields that you have added to the workflow, i.e. all the required fields + the optional fields that you have added to your workflow.
3) After this step everything is good. To test you go in conversation, you take one randomly and click "Select a worfklow to launch manually".
You should see this magnificent lateral bar.
On the right you will see the dynamic fields set up in the initial workflow.
You can decide to
leave them as such,
put some others in its place (be careful however that the field is valid vs. what the API requires),
or put something in your hand (like I did in the video)
Then you click on "Execute"
And it's all good 🎉🎉