How to use world templates in Minecraft bedrock servers

If you mean purchased marketplace content, you can't (legally anyway). Minecraft Marketplace content is encrypted. You need to purchase a realm on the account which owns the market place content to share it via a realm server with friends.

If you mean generic side-loaded .mctemplate files, create a world from the template file first in either Win10, Android or iOS. Then export the save file to the BDS worlds folder and update the server.properties to point to the new save.