It's not possible to directly connect a 90-account to Swish Handel. Instead, Svensk Insamlingskontroll recommends creating a Swish Handel agreement linked to a regular account.
Why doesn't it matter?
When it comes to reporting to Svensk Insamlingskontroll, it essentially doesn't matter whether the money comes in via a 90-account or another account, as they review the financial statements to assess how the organization has handled incoming donations.
Svensk Insamlingskontroll emphasizes that the purpose of a 90-account is only to provide security to the donor at the time of giving. This security is already well provided by displaying their logo on your website to show that you're affiliated with Svensk Insamlingskontroll.
Benefits of Swish Handel
Only the Swish Handel agreement allows you to activate recurring Swish payments. This means that through the Swish Handel agreement, you can receive both one-time donations and monthly donations via Swish through the same agreement.