Ahh, I see now I misunderstood the initial question.
If you are using the ctrlX PLC Engineering REST API, I don't think intellisense will be possible on the PLC project data since the interaction is stateless.
From a brief review of the CODESYS Python script engine it doesn't look like there is native intellisense functionality. I found a forum discussion around this topic and it looks like there may be a workaround, but I have not tried this myself.
CODESYS Python Engine API
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