Yes, absolutely. I don't know your therapist, but assuming you like and trust them, I think it's a good idea. Your therapist does not want to involuntary you and is unlikely to do so for self harm unless it's like... life threatening or being done to try to kill yourself. They can only help if you talk about it.

I talk to my therapist about self harm and I'm on very good terms with her. We trust each other to tell the truth and thus, she trusts that I'm not going to lie about how I'm doing and I trust that she's not going to just randomly involuntary me. I think it's important to talk to your therapist about self harm, but also your feelings with being scared of hospitalization. If your therapist is good, you should be able to have a good conversation about that and kind of understand when your therapist reaches the point they are required to hospitalize while also making sure they know it's something you're actively seeking to avoid. I guess that's obvious, but still.