The link calls out an On-Off switch, so one of the tabs should be marked as the indicator power/ground.
So unless otherwise noted... (don't hold me to it, make sure to check it out) the center tab should be the power in, one of the other two outer tabs will be the power out (when the switch is in the "On" position), the last tab should be the LED power/ground.
NOW, let me stipulate something..., a normal SPST, three tab switch (without an indicator light) would be pwr in the center and loads on either of the two remaining outside tabs dependent on the position of the switch level. Once you add the indicator lights you find that usually one of the outside tabs becomes the pwr for the lights and needs 12V applied BUT there are switches where the light is powered from internal connection and the remanding outside tab is just another load point.
Please see if there is a schematic for the particular switch you have or, do as I do... use a multimeter and check where the signal comes out when you apply power to the center tab and the switch is in the position you want it to be.
Is this confusing because it is to me, ask questions and read the schematics or contact the supplier about the particular switch you have.