I had never heard of iAUC before, but I assume the i stands for incremental, right?

• https://ontosight.ai/glossary/term/Incremental-Auc---iAUC

There are no official SPSS commands to compute iAUC, so you'll probably have to code it yourself if you have to use SPSS. Alternatively, the authors of this article appear to have created a spreadsheet that might give you what you need:

Article Automated Computation of Glycemic Index for Foodstuffs Using...

Good luck.