With so many existing solutions based on the MACD I was shocked not to find this question had already been posted.
Before attempting to build this scan I first checked the built-in scans included with Thinkorswim. I found one named "MACD_Scan" that does the job.
Please consider that the MACD histogram (Diff) follows the relative position of the two lines. When the Value line is above the Average, the histogram is above zero. So if we build a scan for MACD Diff above zero, we achieve exactly what you have requested.
Screenshot below shows that built-in scan configured as you requested. I have included a chart showing one of the ticker symbols from the results list. I had to apply a very restrictive top-level filter to avoid the scan from hitting the time out. So be aware of that when trying to run this scan yourself.