The documentation says /shd custom or from the context menu when unlocked (I have tried both ways) lets us assign a custom name and icon to an otherwise empty button so we can use it as a flyout menu toggle.

Putting an empty macro with for example just a #show <slot> line won't work because hitting that keybind tries to run the macro.

This is for NON native flyout spells.
Say for example I want to put all my teleport tabards in a flyout with the root button working as a toggle.

How?

/shd custom or the context menu just shows a static popup "Finish the operation".
How do I do the operation?

Clicking the empty button apparently 👍🏻