State specific modifications have their own input screens.  These are generally found under State & Local > Modifications > XX Modifications.  In this case, it'd be WI Modifications.

If you scroll down the input screen, you'd find a section called Wisconsin Schedule I Adjustments.  Enter the amount taxable to WI on the line for Certain student loan forgiveness.  This will then flow to both Wisconsin Schedule I, line 1g and line 2 of Form 1.