How do I default a parameter to DateTime.MaxValue in C#?

You can define multiple functions:

public void Problem()
{
     Problem(DateTime.MaxValue);
}
public void Problem(DateTime optional)
{
     // do your stuff here.
}

If you call Problem() (without parameter) that function calls the other function with a parameter.

According to one of your comments, you are trying to make a method with 101 parameters more usable for the callers.
I strongly suggest, that you create a parameter class and initialize the properties of that class with the default values. Provide an overload for your method that accepts only one parameter: The parameter class.
This will really improve the usage of your method, because the user can even reuse its parameter class instance if he needs to change only one parameter.

I would substitute this for something like:

public void Problem(DateTime? optional = null)
{
   DateTime dateTime = optional ?? DateTime.MaxValue
   // Now use dateTime
}

What you're asking to do is simply not possible. DateTime.MaxValue is not a compile-time constant; it's actually a read-only field that is initialized at runtime by a static constructor. That difference becomes quite critical here. Optional parameters require compile-time constants, as they bake the value directly into the code.

However, the real problem is that your method takes 101 parameters. I've never seen anything crying out so loudly for refactoring. My recommendation would be change your method to accept an instance of a class, instead. That will also give you more flexibility in specifying default values for individual properties of the class. In particular, you'll be able to specify values that are not compile-time constants.