How can you play the chords of a ukulele/ guitar?

Usually, whenever I watch a ukulele tutorial, I always see the person playing the chords with their thumb on the side of the fret board. I play it by

Usually, whenever I watch a ukulele tutorial, I always see the person playing the chords with their thumb on the side of the fret board. I play it by putting my thumb behind my fret board, on where I make a chord. It hurts my thumb but I can't do it by the side. Help? Thanks !

or:Usually, whenever I watch a ukulele tutorial, I always see the person playing the chords with their thumb on the side of the fret board. I play it by putting my thumb behind my fret board, on where I make a chord. It hurts my thumb but I can't do it by the side. Help? Thanks !

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