sha = shadow

Add these before the name in the Smodel attribute in the xml to get a superform:
k = Kaio-ken Shadow
s = Super Shadow
s2 = Super Shadow 2
h = Hyper Shadow
m = Majin Shadow
d = Darkspine Shadow
g = Super Shadow God
b = Super Shadow Blue
bk = Super Shadow Blue Kaioken
r = Super Shadow Ros
ui = Ultra Instinct Shadow
Example:
<char model="sha.b3d" smodel="s2son.b3d" bighold="0" metalsteps="0" skates="0" hoverruns="0" motorruns="0" norunsteps="0"/>