Duplicati asks for:
"Bucket Name" - this one is self explanatory.
"Folder Path" - this is the path within the bucket to save your backup data. Use something unique here for every Duplicati backup set. That way you can have multiple backup sets from multiple machines going to the same bucket, if you want. It’s critical that each backup set on each machine store its data in a unique folder.
"B2 Account ID or Application ID" - BackBlaze calls this the “keyID” and it is mostly numbers with some letters. Example: 00123b4567df3c60000000007
"B2 Application Key" - BackBlaze calls this “applicationKey” and it has a slash in the middle. Example: K001mcp0RsiEJe/Viw90bvjkZZPzRSE. Backblaze only shows this to you once at the time you create the application key. If you forget it you’ll have to delete the key and make a new one.