You can create custom typing shortcuts to quickly add common phrases to messages, such as “On my way!” or “See you later!”
Open the Settings app, select General, then Keyboard, and find the Text Replacement section. (Note that for devices running iOS 8 or earlier, the section is called Shortcuts). Tap Add New Shortcut and then enter the phrase and shortcut. Once you’ve added several, you will have to tap the Text Replacement option after Keyboard, then tap the + button to add more.
When typing within apps, you can input the shortcut and the iPad will offer to auto-complete the full phrase. Tap the space bar to complete the phrase and carry on typing.