Action for Standing on a Supporter

I couldn’t find the right turn of phrase to reference the action of the player getting onto a supporter.

For instance, I’m trying to write:

Instead of getting onto the supporter: Do something

And, since I’m here, I might as well say that in this particular case I’d like to be able to do one thing if the player tries to enter the supporter, and another if the player tries to get onto the supporter. Past experience leads me to expect that I couldn’t just say

Instead of/Before entering the supporter: Do something

because the ‘enter the supporter’ rule will never be enacted, since the supporter cannot be entered. I tried using an enterable supporter, but it seemed that “get on thing” was interpreted the same as “get in thing”.

EDIT - I might have found that “…when the player is on the Rocket” works. The original question above is still something useful to me, however.
[s]Alternatively, I could complete my objective if I could test for the player being on something - something like

Flying to Space is a scene.  Flying to Space begins when the player is standing on the Rocket.

But currently Inform is telling me the above isn’t correct.[/s]

Any ideas? Please let me know if I haven’t been clear as to what I’m asking.

I think you’ve overcomplicated things. ENTER and GET ON are both the entering action. You can see this with the ACTIONS debugging command:

The Kitchen is a room.
The table is a supporter in the Kitchen.

(If the table were enterable, both commands would succeed.)