The Bitcoin address in which you are withdrawing your Maidsafecoins MUST be your OWN address, not on Coinbase or another exchange/3rd party service who owns the keys for you.
Its private key is needed when you want to transfer the Maidsafecoins from that bitcoin address to an exchange like Poloniex or Masterxchange, process done through Omniwallet, or when they’ll be exchanged for Safecoins later on this year (not many details at this moment , development still in progress).
As I said no need for your private key now when you withdraw Maidsafecoins to a Bitcoin address but more info on how to find out the private keys of your bitcoin address are on the web or this site.If you’re only interested in keeping them for Safecoin exchange when network is up, you can wait for a step-by-step tutorial then.
EDIT: In this post there’s a very simple explanation on how to find your private keys using a blockchain.info wallet. NEVER expose your private key obviously 
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More info about Maidsafecoin storage here: