Setting up basic concatenation

I am trying to set up a new custom field that concatenates four fields into one. Three are drop down selections and one is a text area comments field. I tired setting up based on some articles I could find but can not get it to read the action in the flow steps. The only official Marketo doc I can find about it does not seems to address what I am trying to do (Create and Use a Concatenated String (Formula) Field - Marketo Docs - Product Docs​ ).

Any help is very much appreciated!