how to raise an event of one form in another form

Not only you cannot raise an even in another form, you cannot raise (invoke) and event nowhere but in the class where the event is declared. This is an important limitation of events compared to regular delegate instances, an important fool-proof feature.

So, technically speaking, I would suggest to use a delegate instead of event, which would work just as well. However, I don't want to advise that. You see, this limitation for events is set for good reasons. As this is a fool-proof feature, it's important to not become a fool by applying some work-around to this limitation. :-)

The thing is: you are approaching to this problem from a wrong way. Instead of explaining the behavior you need, you are putting your vision of how you want to solve the problem in the question. That is not exactly a correct question; and you vision happens to be incorrect. You are almost loosing your change to get a proper advice by formulating a question in a wrong way.

I suppose you simply need to perform the same action with the first form when you press a button on this form, but also on the other form. First, let me tell you that this is a sign of bad design. Secondly, you should talk about the behavior, the action itself, as it has nothing to do with events. You are getting it exactly upside down. You need to do the opposite thing: you need to have an instance method of first form, so you can call this method from two different places: from some event handler in form one and another event handler in form two. Isn't that trivial?

Now, it leave for yet another problem: how to pass a reference to form one to the form two? Well, you can always do it, but won't it be nice or not? You might be passing too much access to another form. This is the popular question about form collaboration. The most robust solution is implementation of an appropriate interface in form class and passing the interface reference instead of reference to a "whole instance" of a Form. Please see my past solution for more detail: How to copy all the items between listboxes in two forms[^].

—SA