Upon set up of a trustless masternode, then you have saved the presigned transactions that allows you to withdraw.
You can broadcast the transaction from Dash Core, but you can also just do it from a few block explorer websites:
The concept is the same, you just paste your transaction into the text field and push the broadcast button.
Dash Core
Using Dash Core, you just go into the concole and type:
sendrawtransaction INSERT_YOUR_RAW_TRANSACTION_HERE