Subrogation claim?

In October this year I did physical therapy for my neck/shoulder/arm as I was diagnosed with TOS after I started working a repetitive movement required job.
And my insurance just sent me a subrogation form to fill out. I'm a bit confused, because true the work did develop and aggravate my symptoms. But I won't say the work 100% caused my injury/diagnosis as I do have some abnormality (cervical rib) -which you will have higher chance to get TOS if you have-.
However I never knew I had extra rib (till I did x-ray after the intense pain cause the work), and all the pain I done PT for was because of the work (I still struggle with that).
My employer knew I was in pain since August and they refused my accommodations in October, and just recently (end of November), they approved for "leave of absence".
So I'm not sure, what are my options in this situation? Will I be charged part of the claim which is 3k, if I didn't fill the form?
P.S: it's my first time experiencing all this, so excuse my ignorance. And I'm in the US.

submitted by /u/BeyourselfA
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