Under options you can allow Null Data
Allow data to be sent within the transaction and stored in the blockchain by using OP_RETURN.
When using this option you may enter a hex string or address into the address field on the output tab.
Assuming you have a script to generate this hex string, Coinb.in can be used to create a counterparty transaction, right?
The outputs will then be
- The opreturn, amount 0
- Your address, current balance minus fee (and dust)
- If a send transaction, some dust to the destination