In Base Report Design how to I enter a formula here:

I want to put in something like for example CONCAT(FieldName1, ’ ', FieldName2) in for the Data field. It appears that it will take this from the = sign here
but I can’t figure out how to enter a formula.
I know that I can do this in the underlying query, but it just seemed like I was also able to do this in the Report design editor.
(here on the left)