Where to enter this in NextChapter: New Unsecured Nonpriority Claim > Claim is an unsecured portion of a secured claim checkbox

If you have a claim that is an unsecured portion of a secured claim, first put the secured portion of the claim into the Secured Claims section

After you have entered the secured claim, you can enter a new unsecured claim into Unsecured Nonpriority Claims.  You can attach it to the secured claim by selecting the box labeled Claim is an unsecured portion of a secured claim and selecting the appropriate claim from the dropdown