At IB for equities I can use the order type MKT/OPG to get the opening auction price. On the ThinkOrSwim platform all I see is ability to do a Market order, there is no 'Opening' selection. Question: at ThinkOrSwim if you enter a Market order for a stock before the market opens, does it automatically become part of the opening auction?
Not being helpful at all but you'd be shocked how many brokers still don't have the auction orders at all.
According to ToS support they currently only support Market-On-Close and Limit-On-Close. Doing a market order before the open doesn't guarantee that you get the auction price. I think if you requested they add MOO they probably would. Based on what I've read the rules for MOO are now more formal than they used to be.