Ball in magnetic field, not understanding boundary condition

So I am reading solution of following exercise:

A conducting sphere with radius $R$ moves with constant velocity $v=ve_x$ inside a constant magnetic field $B=Be_y$. Find the induced charge distribution on the sphere to 1st order in $v/c$ in the laboratory inertial reference frame.
Which is on page:
https://www.physicsforums.com/threads/moving-sphere-in-magnetic-field.825426/
So the personal is this topic solves laplace equation with condition that potenial w infinity must go to $-rgamma frac{v}{c}costheta$ I don’t get why.