Somewhere in that link is another link to an online tool which can help you create a custom coordinate system using the Esri WKT shown on this page: https://epsg.io/7792.
It seems like you have stored the coordinate system in ...\Documents\FME\CoordinateSystems\CoordinateSystems, can you move the .fme file to the first CoordinateSystems folder? If that doesn't work you can double check if the folder you put it in is a shared folder (tools > FME options > default paths > shared FME folders)
Also, you have to restart FME for the coordinate system to appear in the gallery.