Turn on triplet mode (press g).
Select 16th notes (semiquavers) (press 6).
Insert 6 notes with the keyboard. (Use the following keys: qweruio, asdfjkl, zxcvbnm)
Confirm two 16th note triplets.
Select 8th notes (quavers) (press 8).
Insert 3 notes with the keyboard.
Confirm one 8th note triplet.
Select quarter notes (crotchets) (press 4)
Insert 3 notes with the keyboard.
Confirm one quarter note triplet.
That should fill the first bar.