If Halo Fountain becomes a creature or a creature gains the activated abilities of Halo Fountain (e.g. Scheming Fence), you can activate the top-most ability by paying 1 Mana, tapping the Fountain (or the Fence), then untapping it again.

The problem here is that MTGO will only let me activate that ability if I have another already tapped creature on the battlefield.

I am unsure if this is a bug or working as intended, but, seeing as I can pay all the costs of the activated ability, I thought I should be able to activate it.