You can deposit 100 USD with AMP and start a Rithmic account through them. You dont need to take any trades, just keep a minimum of 100 USD on the account.
Also this is taken directly from Rithmic webpage.
Is it possible to use Rithmic's software without a Broker?
Yes, you can purchase our data feed directly without a broker. However, you might find it cheaper to open an account with a broker which offers Rithmic.
For a list of brokers that offer the use of Rithmic’s software to their account holders, please visit
Introducing Brokers (rithmic.com).