i want to create custom workflow action for SharePoint designer workflow which will copy documents across sites. i want to deploy this action on SharePoint online.
I would suggest using Flow for this kind of tasks in Office 365 as it's the new trend in doing these things. You can create a new item anywhere, when an item is added to any list. You can check some templates you can start from here:
To do it using workflows in O365, you would use REST to create an item in the desired list, an example here to help you out: