Any tips on how to get someone with dementia to use sunscreen?

Some of the Japanese and Korean sunscreens are very much like face lotion, so if your person uses face lotion already you could perhaps have them switch to it. Offhand, CosRX and Nivea watergel are both nice with an SPF of 50.
If you're looking for a full on 'sit in the sun' application, it seems like that would be an uphill battle unless they're willing.

Let me put it to you this way: I can't even get my mother with dementia to remember what I told her 3 minutes earlier, never mind to put on sunscreen! I can tell her she lives in her Memory Care suite 3x and two minutes later, she'll tell me she's staying 'in the hotel' tonight and won't be home.

Manage your expectations with a dementia sufferer. Better yet, stop having expectations and then you won't be disappointed!

Other than you or someone else putting it on her, it won't happen. You can't expect someone with dementia to remember anything really, so it may be time to lower your expectations. If she has vascular dementia like your profile states she will more than likely die from that quicker than any skin cancer she may get from not putting on her sunscreen.
You have to learn to pick your battles with someone with dementia, as they're not all worth fighting. Best wishes.

“I thought you might enjoy trying this lotion (cream, moisturizer, etc.). I use it often and my skin has never looked better. Your skin is lovely”.

Why doesn’t she want to “use sunscreen? If she has dementia, she won’t remember to use it regularly, but if you leave it handy and mention it casually, and speak about how nice her skin looks, she may at least remember nice in a while.

Always be sure to pick your battles.