I don’t see why you would need an indulgence. You’re still in this Earthly life. However, I think you mean penance for your sins. In order to get the indulgence for someone, you have to obviously imply to whom and I would also share the indulgence for others that may not have been prayed for by anyone else since we do not know who exactly is in Purgatory or if a specific person is in Purgatory. You never know how much power your indulgence has, it may be enough to indulge others as well or at least partially. Also remember that the more Grace and Faith you have when praying, the more effective your indulgence will be. I suggest a good confession and reception of the Eucharist first. Also, use the Bible in your prayer wether it be the Rosary or etc.
I’m not sure in what way you did not ask them to be applied. To gain an indulgence to begin with, you have to have made or have a general or habitual intention to gain them. So one would not gain them at all if one didn’t know about them.