How to set dynamic variables in AWS Step Functions

I want to assign a variable from one of the state in AWS Step functions. The value of this variable has to be dynamic. I tried various syntax (as suggested in AWS documentation) to achieve this. These are shared below.

"Assign": {
          "index": "$states.input.index", // read inputs from states variable
          "test": "This works fine"
        }

Or

"Assign": {
          "index.$": "$.input.index", // state gets input object in this structure
          "test": "This works fine"
        }

Or

"Assign": {
          "index": "{% $states.input.index %}", // JSONat format
          "test": "This works fine"
        }

None of them seem to work and I end up getting string literal value for the dynamic expression instead of its evaluated value. Can someone help with what am I missing here?