Hi partner,
Since you are using Online now, which client are you working?UCI or Classic client?
If you are using Classic Client, it shows the same as on-premise.

If you are using UCI, it is a new client since V9.0, and there is no shortcuts to go to the form editor.

I suggest that you could open the form editor in another tab of the browser for more convenient.
There are 2 ways to open form editor in UCI.
1.Open the "Advanced Settings" ->Cusotmizations->Customize the System


2.Go to My Apps and open the app designer.


Find the form and open the form editor.
But unfortunately, we could not open the form editor directly in UCI form like On-premise now. We should use the above 2 ways to open form editor.
Best Regards,
Leo