I have learned that you are able to put buttons in your custom class using a simple method explained below.

Button Code
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B = B button
A = A button
D = Y button
C = X button
F = RB (Right Bumper)
E = LB (Left Bumper)
S = RT (Right Trigger)
R = LT (Left Trigger)
Q = RS (Right Stick)
P = LS (Left Stick)
N = Start button
O = Back button
T = Dpad up
U = Dpad down
W = Dpad right
V = Dpad left
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Alright these are all of the codes you'll want to use when making your custom class.
1) Type in the code into your custom class.
2) Hit enter >.<
3) Go back into your Custom Class
4) Hit enter.
5) Add more buttons, if you need to.