The Bm7 is another simple barre chord, there will just be a little more pressure on your thumb to hold the ukulele upright. Place your index finger across the second fret, with the top joint slightly hanging over. When you press down on all four strings, they should play with no muted sound. You may need to make small adjustments with the index finger until you find just the right spot. Barring just one fret can be a little awkward, thus the extra grip in the thumb.