My first suggestion to you is to use the inputs instead, just map the particular key you're using to the default value for a button in the controls you aren't using (Look in Controls.cs). That is the suggested way of getting input from the player, since the player can then customize the controls.
However, if you have only one place that this key is used, and it's a special case, or you just don't have any inputs left in the controls, or you have some other reason you want to use IsKeyPressed, you can make up your own "InitialOnly" manually.
I'm assuming these rules are being executed on a sprite. First, create a property on the sprite called wasPressed. Then create this control structure using the rules:
if IsKeyPressed(your key)
if wasPressed == 0
(Do whatever you want to do)
end if
wasPressed = 1
else
wasPressed = 0
end if