There are two easy ways to add an intro to your song.
Option 1: The ending of the song works as an intro.

Tap and hold for example 4 or 8 measures from the end of the song and play from there. Remember to add one chorus to the total count as the first chorus will be the one that serves as an intro.

Option 2: Insert space at the beginning of the chart and enter the chords for the intro.
Insert the necessary space before your first bar at the top of your chart by tapping on the 'Insert space' button
For example to add 4 bars, you need to tap the 'Insert space' button 16 times (4 spaces x 4 bars).
Add your time signature, bar lines (start and end with double bar lines) and rehearsal marks
('in' for the intro and 'A' for the song in case it wasn't there already). Don't forget to remove the time signature from the 'A' section by tapping on the same time signature in the Editor.