Technically speaking, there is no "Economic Calendar" in Thinkorswim. Under the MarketWatch tab we find an option called "Calendar" displaying corporate actions. This is a completely separate tool from what you see displayed at the bottom of the chart. It's very important to make that distinction and explain the differences to avoid confusing everyone viewing this post.
The icons you see at the bottom of the chart displaying stock splits, earnings and dividends is actually called "Corporate Actions". This is a chart setting and it is either on or off. There are no options to modify them beyond that.
Screenshot below shows where to find the chart setting as well as where the corporate actions appear at the bottom of the chart.
Please let me know if you actually had something else in mind here and I will try to address that as well.