In SSRS Visual Studio 2017, where do I find the place to the "=IIF" statements for a tablix?

There a way, in SSRS to have just one row on a table (Tablix) highlighted if that one row has important information for the user.
Let's say, for example, I wanted the first row in bold, I could set the background color property or font weight based on whatever my criteria is. I can use ROW_NUMBER() to check if it's the first row. Something like
=IIF(ROWNUMBER("yourDatasetName") =1, "#FF0000", Nothing)
But I forgot where I can navigate to this point where I put in this information. Please help.

Select the tablix row , go to properties window,click background color and then click expressions