Match Game / Blankety Blank has two types of prompts - both of which use the Open Question type.
In the main round, the prompts are short scenarios, in which a word of your choosing is blanked out for the player to fill in. To indicate a blank, you can use the word BLANK in the body of the prompt, or use underscores (i.e. _____) to indicate a blank space - either will work. There must be a blank somewhere in the prompt for it to be accepted by the game.
In the Audience Match and Super Match rounds, the prompts are two word phrases, of which one is a blank for the player to fill in (e.g. HOUSE _____). Again, you can either indicate a blank with the word BLANK or a series of underscores.