How can I claim back tax paid on imputed income when my LDA is my tax dependent

Back in 2017 I added my mother to my medical insurance as my LDA. Since then I have been paying the insurance premium for my mother was considered as extra taxable income (imputed income). I claim my mother as tax dependent and I didn’t know if your LDA as tax dependent you don’t have to pay the premium tax. My employer says there's nothing he can do on his side jus to make sure that I’m not going to paid the premium tax going forward . How can I claim the extra tax I paid for 2017, 2018 and what I need to do for this year?

Thank you so much,

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