I would argue that that which makes me 'me' is also that which makes my will.
I guess we could go with that. But 'you' are always changing. The obvious way is that you are getting older. You're obviously not the same person were when you were a child, or an adolescent, or teenager.
I don't know if you're a fan of Doctor Who, but in the series, the Doctor keeps an old dog-eared diary documenting his life. It begins pristine and new, and then slowly becomes tattered and old, full of stories and notes, and drawings of the people he's met.
Think of your life like that diary. When it's new there's not much in it except a listing of your DNA, documenting what it physically means to be you. From the color of your eyes to your propensity to be shy. It's all there in that long strip of DNA. But it doesn't really know how to react to questions of morality or civility. In response to prior events it just cries, or sleeps, or poops. It doesn't really have anything that we could rightly call a '
will'. None-the-less it's you.
But time goes by and the entries in your diary grow. You develop a sense of the things that you like and the things that you don't. Yet it's still a work in progress, so prior events and DNA still dictate much of how you respond. But you've got the makings of a will, and more of a sense of what it means to be you. You're not there yet, but you're getting there.
At some point however the diary begins to fill up, such that the prior events that actually matter aren't the one's outside the diary or in the DNA, instead they're the ones inside the diary. They're the memories, and experiences, and sense of morality that truly define who you are. Those are the prior events that matter, and those prior events are you. You're not just a physical thing anymore, you're an intangible thing recorded in a diary that's your life.
So yes, you're still governed by prior events, but the prior events that matter are the ones in that diary... they're you. I have a will, and how I came to have that will doesn't really matter as much as the fact that I have it. Because thanks to wills men such as John Proctor can choose not to sign a paper.
There's a point at which I am what I am, and if I want to change, then I can change, even if it's only in the desire to do so, and nothing else. Ultimately I'll do me, and I'll gladly let you do you. But when it comes to will, and specifically free will, it's not something that comes whole and complete from inception. It grows, and so yes, it has a cause. Yet at some point what's inside that diary becomes more important than what's outside that diary... and what's inside that diary is you.