I need to have a
Project ID column in a project named list.
The format would be
PROJ0001 PROJ0010 PROJ0099 PROJ0100
and so on. These I'd will be auto populated while creating any new item in the list.
How can this be done in SharePoint Online?
The ID value is a sequential numbering for List Items.
You can use the Calculated Column Formula:
=TEXT( [ID] , "PROJ0000" )
Only one caveat: Newly created items have a 0 value for the ID column, because the ID value is determined after all Formulas have been calculated.
That is why you don't see ID listed as usable column when editting a Formula
Trick is to create a (on new item) Workflow which writes the ID value to a second column MyID so you can use:
=TEXT( [MyID] , "PROJ0000" )
You can trigger the ID update without a Workflow, but that is a more advanced usage.