ModelBuilder: Fields not available when selecting attributes as a parameter

[LDort]

Hello,

I am creating a tool to be used in the Geoprocessing pane that has data brought in from my organizations Portal connection. I need attributes to be selectable in the geoprocessing pane so I created a variable (an expression) and set it as a parameter. The users of this tool can write an SQL expression to select their attributes but I am wondering if there is a way to have the fields (and possibly the fields unique values) show up in the drop down menu, rather than it be blank. 

I have tried creating a .lyrx template but it seems to just affect the symbology rather than giving me the data's fields. Any guidance would be greatly appreciated.

[DuncanHornby]

It's not entirely clear what you are asking for, always good to include images in your question rather than blocks of text...

That said is this what you want? I expose the two parameters and when I run it as a tool I get the default drop downs, as if I was running the tool by itself.