FilterCriterion and a customised fetch action

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When you use the customize fetch action you no longer have a filterCriterion parameter. I know how to build the query parameter manually (say for the Sales Cloud REST API) but VBCS seems to have some weird quoting requirements that's making it difficult (I can query in PostMan but not with the same parameter in VBCS, I have to add ' or " everywhere). I figure that the FilterCriterion parameter takes care of this for me, but it's not clear how to call that when you've customised? Thoughts?