Anyone and idea what to fill in at the field Default Sales TAX Ids to automate magento 2 order into wave see screenshot.
Unfortunately, this integration is built by Integromat, and we don't have any knowledge of how it is designed, or how to resolve your issue.
It sounds like you may need to map (convert) a tax record as Magento understands to an equivalent Sales Tax within Wave and use the Wave value within "Default Sales Tax Ids". See how to add a sales tax guide.
I suggest you reach out to the Integromat support team for guidance. If they themselves are having any issues with their connection to Wave, we're of course ready to help them resolve that for you and all Integromat customers.
Sorry I can't be more directly helpful.