Q:

C# Apply Interface to a Form Class

How can I apply an Interface to a form class
partial class Form1 : Form, InterfaceA

Is this correct? 
Basically I would like to implement an Interface on a form.  
How To ....

A:

A Form is just a class (that subclasses System. Windows. Forms. Form), so yes - standard syntax  is fine, as you have it. 
Edit: As to your partial class part of the question, no, you need only declare that you implement the interface once.  From MSDN...

If any of the parts are declared abstract, then the entire type is considered abstract.  If any of the parts are declared sealed, then the entire type is considered sealed.  If any of the parts declare a base type, then the entire type inherits that class. 

Remember, there's no magic in forms or partial classes.  C#/.Net is one of the few Microsoft projects that's wizardry free - it really does tend to behave the way you think it should.