C#: How to set default value for a property in a partial class?

I'm very new to C# so please bear with me...

I'm implementing a partial class, and would like to add two properties like so:

public partial class SomeModel
{
    public bool IsSomething { get; set; }
    public List<string> SomeList { get; set; }

    ... Additional methods using the above data members ...
}

I would like to initialize both data members: IsSomething to True and SomeList to new List<string>(). Normally I would do it in a constructor, however because it's a partial class I don't want to touch the constructor (should I?).

What's the best way to achieve this?

Thanks

PS I'm working in ASP.NET MVC, adding functionality to a a certain model, hence the partial class.