My example was based on a custom trigger. The userId is passed from the event (from the webhook created by the custom trigger) to the action. For the dependency you can select the tigger output User. As there is no trigger input User the user will not have to fill in the userId.
With a built-in trigger you can still select the trigger > userId (if the built-in trigger supports that field). Most of the built-in triggers sends the userId in their output, so you should be able to selecte trigger > userId as the dependency when you create your recipe.
You can easilly check that by taking the built-in trigger you want and use the action “notify (with item and user)”. If you can select a userId in that action, the tigger apparently sends the userId so you should eb able to select that.