n8n is super cool but I am not understanding something, why the start node (the one with the green arrow) is not having a parameter that could be the initial data to test the flow. This will avoid having for instance to use PostMan or Curl to trigger some flows. Did I missed something?
You can use the Set Node which sounds like it will do what you are after so go from the Start to Set then you can move on from there or if you have something specific you want to emulate there is always the function option.