Hi Mayur,
Did you want to get selected item in the custom action in context menu of a SharePoint list ?
If that is the case, I would suggest you can use JavaScript Client Object Model to get selected items in SharePoint Online and then call the JavaScript in custom action to achieve it.
Here are some detailed code demos for your reference:
Get Selected items in a list sharepoint online
Referencing JS files using CustomAction element
Thanks
Best Regards
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