Was able to figure it out. Had to add -
<HotButton item="HB5_Button1">
<EQType>hotbutton/Bar 4/0</EQType>
<Font>1</Font>
<RelativePosition>true</RelativePosition>
<Location>
<X>117</X>
<Y>52</Y>
</Location>
<Size>
<CX>20</CX>
<CY>20</CY>
</Size>
<Text></Text>
<TextColor>
<R>255</R>
<G>255</G>
<B>255</B>
</TextColor>
<DecalSize>
<CX>40</CX>
<CY>40</CY>
</DecalSize>
<SpellGem>HB_SpellGem</SpellGem>
<InvSlot>HB_InvSlot</InvSlot>
<ButtonDrawTemplate>
<Normal>A_HotButton1Normal</Normal>
<Pressed>A_HotButton1Pressed</Pressed>
<Flyby>A_HotButton1Flyby</Flyby>
<Disabled>A_HotButton1Disabled</Disabled>
<PressedFlyby>A_HotButton1PressedFlyby</PressedFlyby>
</ButtonDrawTemplate>
</HotButton>
Then in my CombatAbilityWnd, I added -
<Pieces>HotButton:HB5_Button1</Pieces>