You can send a gift subscription to yourself by tapping on the profile tab (at the bottom right of the toolbar), and then click on the 'gift' icon on your profile.
You will be taken to the gift store and the top of the window will indicate that you are sending a gift to your own nickname.
Scroll down and you will see the 'subscription' gifts listed, along with their cost in credits. Tap on the gift to select which subscription you would like. Gift subscriptions can be given for a one week or one month duration. Year long gifts are not available when buying with credits.
The next screen will ask you to confirm your purchase. Tap on the 'purchase gift' button to send the subscription to yourself.