There is no such thing as a separate AGI for each spouse on a joint tax return. Your own AGI is the amount on Form 1040 line 11 of the tax return that you filed. Your spouse's AGI is the same. There is no standard way to allocate part of it to each spouse.
As others have suggested, ask the loan servicer what you should enter on their form. It might be necessary to file as married filing separately in order to reduce the loan payments on an income-based repayment plan. Many people in your situation do that.
HOWEVER, it's too late to change your 2022 (or earlier) tax return to married filing separately. You are not allowed to change from joint to separate returns after the due date of the return, which has now passed. For 2023 you have time to collect more information and figure out whether the reduction in the loan payments is worth the extra tax you will have to pay if you file separately. Keep in mind that income-based repayment does not reduce the total amount that you owe on the loan. It only reduces the amount you have to pay each month, which means it takes longer to pay off the loan.
