Because you have put spoiler in the subject box it would be ok to place it in here, if your still wanting to use the invisable text heres how you do it.
Reply to the thread, the quick reply at the bottom doesnt work, write your text explaining what you want then highlight the section of text you want to be invisable then select the color change button and click on the color that is the same color as the background, NOT WHITE its the color that is 5 from the left on the top row, code for the color is #FFFFCC once youve done that preview just to make sure.
hope this helps.