Originally Posted by ShakuniMama

That is correct.
Once you make a purchase you will see your starwood points balance under your account at Amex Cards. Click on statement and your statement will show your starwood account number. Note down that number and go to spg.com to "activate" your spg account. Alternatively you can call them to get your account number.
Do not register for a new spg account since an spg account number has already been assigned to you.