I've never posted a PNR for Trip Diary or for analysis. But here's how it must be working:
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> Go to PNR Forum
> Post PNR
> Click on "Prediction/Analysis/Trip Diary"
>...
more... A Window will appear as in Re: 488113-3 above
> Key-in the required details
> Tick the "I wish to register this PNR in the Trip Diary"
> Click on Post PNR for Predtion/Trip Diary"
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Two birds killed in one stone