Take a static variable in FormB which will hold the reference of currently opened form:
Code in FormB:
public class FormB : ....
public static FormB frmCurrentInstance;
frmCurrentInstance = this;
In FormA by using the static variable of FormB you can access the current instance of FormB, and using that instance you can invoke the method to perform desired action.
Access the PerformInterfaceMethod() using static variable of FormB.
Hope this solves your requirement.