Before you start
Things you need to know:
Product recommendations with a custom tax rate will overwrite any tax settings customised at catalog level.
The catalog default tax rate only applies to product recommendation blocks.
Set the default tax rate for a catalog
Go to Content > Products > Catalogs.
Find the catalog you want to set the default tax rate for, then select Properties. This opens the Edit Insight properties side panel.
Under the heading Tax settings, for Prices include tax, select No.
For Default text rate (%), enter your catalog's default tax rate as a percentage.
Product recommendation block
When you add a product recommendation block built using the catalog in question to a campaign, you will now see the base tax rate under Settings > Fields. By default it will show the figure you entered for the catalog but it's possible to edit the percentage figure in the % base tax rate field for the specific product recommendation. The tax rate will be added to the Price and Special Price of your products.