You could start like this:
method = POST
URL = https://test.bigchainb.com/api/v1/transactions?mode=commit
The final, signed (fulfilled) transaction goes in the body, but I'm not sure what format Postman expects it in. Maybe a Unicode JSON string?
To construct a valid signed transaction, you should probably use one of the BigchainDB drivers, and if you're doing that, then why not also use the same driver to POST the transaction to the BigchainDB Testnet? Here's a list of drivers: