Use inverse kinematics (IK) when making an animation for a custom rig?

I want to use inverse kinematics to animate this spider alien thing that i made, because it is way easier to use that method of animating. Now with normal r15 and probably r6 rigs theres always this IK button in the animation editor that appears where you can enable inverse kinematics. However this does not appear when i want to use IK on my custom rig. How do i make it show anyways and use it when animating this custom rig?

This is what the spider looks like and the animation editor that shows up.

And here on this r15 rig (ignore that its mao) an IK button appears, next to the name of the model.