Files rename

Hi,
am trying to rename series of files as following
Hello.txt -to- hello123.txt
test.txt -to- test123.txt

any idea how to do add text after the file name and bfore the extentions. I tried this cod but it does not work well
for /f %%i in (‘dir /b /a-d *.txt’) do @ren %%i %%i.txt.

Thanks