To the best of my knowledge, when you buy a starter pack, the Omega time is automatically added to your account at the time of purchase. You don’t choose to ‘activate’ the Omega time later. Some specific items in a Starter pack (like a booster), you do choose when and which character to activate them on, but not the Omega time.
At least that’s my understanding, and various posts indicate it as well:
If anyone has more recent or more specific information(eg. Steam-related, since I buy direct from EVE Online store), please update the thread.