When you click on an Adobe Sign link for the first time, it will prompt you to sign in to an Adobe account. This will take you to Adobe's sign-in page, where you can enter your SPU email.
Once you enter your SPU email, it will ask you if you would like to use a "Personal" account or a "Company or School Account." Click on "Company or School Account."
After choosing "Company or School Account," you will then be taken to the SPU SSO page, where you will be prompted to enter your SPU credentials. After entering your credentials here, you will be able to fill out your form using Adobe Acrobat Sign.