You can insert system variables into either the Job or the File Event that you're using for the rename. I'll admit, however, that I've never tried doing this with the File Events. It should still be able to take the TimeStamp system variable, however:
<SysTime.h:mm AM/PM>
That's only one example of the available formatting. In the case of using a File Event, set it so that the Monitor points at the abcd.txt file, and the Disposition uses an Operation of "Move file to target directory", set so that the location is the same, but the file name is abcd_<SysTime.h:mm AM/PM>.txt
I've never tried that, however. I'm curious as to whether or not it would work.
Alternatively, you could create a Job which uses Cmd.exe as its Command, and Command Parameters as follows:
/C ren abcd.txt abcd_<SysTime.h:mm AM/PM>.txt
The Working Directory would be set to the location of the abcd.txt file.