There are two scenarios in which you can use a limit order as a market order:

• For buying: If you want to buy any stock or derivative, place a limit order at a price which is above the last traded price or above the highest ask price in the bid-ask spread. 
• For selling: If you want to sell any stock or derivative, place a limit order at a price which is below the last traded price or below the lowest bid price in the bid-ask spread.

By doing so your limit order will get the first priority and you will be able to execute your order at the highest price at which the seller or buyer is ready to sell or buy shares.