Try using the lowest notes of the piano roll. Of course those are the purple ones all the way to the left.
If you select those left most notes, and select the ones that come on beat 1 every 4 beats, you should have a good, simple bass to go with the melody.
From the beginning you would get the notes:
B E A F# | B E A F# | ...
I'm not sure if this could be a source of trouble for you, but the piano role gives notes Dd, F#, and Ab which is terrible to read and use for harmonization. These notes should be C#, F#, and G# putting it clearly in A major. With that key we can start with chords
Bm E A F# but a quick review of the piano role seems to use a lot of 'sus' chords and ninth chords.
As Todd mentioned in comments it's hard to summarize how to harmonize a melody in a short SE answer. But, many harmony textbooks include chapters about harmonizing a given bass or melody. Try to find some textbooks to learn those methods. I can tell you that you have to understand keys, meter, non-chord tones, harmonic rhythm, and common chord progressions to create a good harmonization. If any of those elements is not familiar to you, you will need to devote some time to learning them. The following web site gives a descent overview of harmonization:
One caveat: pop and rock music songs often have melodies that don't "agree" with the chords in a standard sense. So you need to apply the textbook methods with some flexibility.