First, you should file Iowa Form 44-016 with your employer so that they cease withholding Iowa taxes. Here's a link to that form:
https://revenue.iowa.gov/media/2318/download?inline
Illinois will not give you a credit for mistakenly withheld Iowa taxes. In order to have those taxes refunded to you, you must file a non-resident Iowa tax return, on which you show the withheld Iowa taxes but claim zero Iowa income. See Tax Withheld in Error in this Iowa tax reference:
https://revenue.iowa.gov/taxes/tax-guidance/individual-income-tax/1040-expanded-instructions/note-ad...
Your income, including your Iowa earnings, is 100% taxable by your home state of Illinois.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
