Very soon (usually within minutes) after you purchase your Premiere membership, a key will be sent to the email address you provided at that time (for PayPal, it is the primary email of the PayPal account used). You simply copy and paste this key into your Emby Server Dashboard under the Emby Premiere section (click "Emby Premiere" in the side-bar menu).
If you used an app to purchase your subscription, your key will still be sent to you via email, but it also should be entered into your server automatically right after the purchase so you won't have to wait for the email to arrive.
If you do not receive your key, check all of your spam traps as sometimes those think these automatic emails are spam. You can cause the key to re-send with the "Forgot Key" feature on the same page of your server dashboard.
If you still have trouble obtaining your key, please post a topic in the Emby Premiere section of our community forum.