Send a mail to corporate.ffs.tcs.com with subject line 'Experience Letter for Immigration purpose' , They will send you a sample template, you need to fill it and send it back. They will ask for your previous supervisor, visa application number, attorney number etc. I replied stating, I do not have an attorney and just quoted a random number as application#. Also my supervisor was still working there, if supervisor has left TCS, then anyone above your previous designation in TCS can approve the request.
TCS is very good and helpful, when replying the guys will have their direct phone number on their signature, in case of any delay call them or ask me for help, I will find their number in my old emails.