How do I use my partner's previous year's unclaimed donations on my return?

My partner has unclaimed donations from several previous years but his net income this year was zero. CRA form 5000-s9-20e makes it very clear that either partner can claim unused donations from previous years:

 

Include the eligible amount of all the donations you want to claim this year that you or your spouse or common-law partner made in 2020 or in any of the previous 5 years that have not been claimed before.

 

However, although TurboTax knows about these unclaimed donations, it won't let me claim more than this year's donations on my return. How do I get around this?