  In this work, we demonstrate field-induced Bose-Einstein condensation (BEC)
in the organic compound NiCl$_2$-4SC(NH$_2$)$_2$ using AC susceptibility
measurements down to 1 mK. The Ni $\textsl{S}$=1 spins exhibit 3D $\mathit{XY}$
antiferromagnetism between a lower critical field $H_{c1} \sim$ 2 T and a upper
critical field $H_{c2} \sim$ 12 T. The results show a power-law temperature
dependence of the phase transition line $H_{c1} (T) -H_{c1} (0) = aT^{\alpha}$
with $\alpha =$ 1.47$\pm$0.10 and $H_{c1} (0) = 2.053$ T, consistent with the
3D Bose-Einstein Condensation universality class. An abnormal change was found
in the phase boundary near $H_{c2}$ at approximately 150 mK.
