No, it's not possible to activate your Cash App card without having received it in the mail yet. You need to have the physical card in hand to activate it through the Cash App. Here are the steps to activate your Cash App card:

1. Open the Cash App on your mobile device.
2. Tap the "Cash Card" icon at the bottom of the screen.
3. Tap the "Activate Cash Card" button.
4. Follow the on-screen instructions to activate your card by scanning the QR code on the front of the card.

If you have not received your Cash App card in the mail yet, you will need to wait until it arrives to activate it. If you have any issues with receiving your card, you should contact Cash App support for assistance.