You are basically correct, except that it should be qAttributeExpressions (with the s at the end). The help seems to have this wrong.
The contents of that array is NxAttrExprDef: https://help.qlik.com/en-US/sense-developer/3.0/Subsystems/EngineAPI/Content/Structs/NxAttrExprDef.htm
Which can contain an simple expression or a libraryID.
so your end result should look something like:
"qExpression": "Avg([Sales Cost Amount])"
That should work, but check the resulting dimensionDef if it doesn't.
(Sense client dev)