Hi Arun,
Overriding the behaviour when a row in a sub-grid is double-clicked is not supported. I can see two workarounds that you could investigate:
1. Add a ribbon button to the sub-grid that launches the custom web page passing the selected row Id in to it. This would be the most supported way of doing this but would not prevent the user opening the form directly.
2. Add OnLoad javascript onto the CRM form to redirect to your custom page when it is opened. This would prevent the user ever accessing the form directly but would be wasteful in that it would unnecessarily load the form, redirect and load the form again.
Michael Palmer
xRMPalmer
@MJFPalmer
Rockstar365