Custom Export help

Ok I must be dumb or something. I have been working for hours on how to change the fields for custom export. Everyone makes mention of file name this or that, I need simple instructions. I click on the little icon in the export window to create a new or edit a current export and all i get is a pop up to select the APP i want to use, and the only option is the Microsoft store and it lists nothing available. So maybe i am doing something wrong here or I just dont get it. I need simple instructions. Like click export, now click this, type this here, look for this, etc. Code examples dont help me if i dont even know where to put them.

The export scripts are plain text files.
So please use the Windows functions to associate files of the type mte with (the Windows plain text) Editor.
You will then see the contents of an export script.