Static Study - How to select a coordinate system?

Hi.

I am trying to create a dummy study that will show me what is the deformation of a tube when force is applied on the midle of it so i:

1- Created/Sketched a round tube of 20in (2 circles - and extruded boss, very simple) OK

2- Created a static study  and applied fixtures on the tube ends. OK

3- Clicked External Loads ->  Force. OK

4- Clicked the tube selecting Face1 that is the entire tube. OK

5- Selected nonuniform distribution - OK

6-> Now it says i have to select the coordinate system but i have no idea on how to do it????

More info:

It is showing the X,Y,Z arrows on the left bottom corner of the window.

I tried to select "Origin" from the menu tree with no success.

How to select it?

SolidworksSimulation