The += operator is implicitly defined like this: a += b turns into a = a + b;, same with the -= operator.
(caveat: as Jeppe pointed out, if a is an expression, it is only evaluated once if you use a+=b, but twice with a=a+b)
You cannot overload the += and -= operator separately. Any type that supports the + operator also supports +=. You can add support for += and -=to your own types by overloading + and -.
There is however one exception hard coded into c#, which you have discovered:
Events have a += and -= operator, which adds and removes an event handler to the list of subscribed event handlers. Despite this, they do not support the + and - operators.
This is not something you can do for your own classes with regular operator overloading.