Margin+ is by invitation only. Those eligible for it can submit their application on the Margin+ page by clicking on Get Margin+ Now:
How long does it take to process Margin+ application?
You will get an update on your application within 72 hours. If your application is approved and you meet the minimum balance requirements, you’ll have your Margin+ funds credited to your BitMEX account shortly.
Requirements after Margin+ Approval
Once you’re in the program, you must meet the defined trading obligations – otherwise, an extension fee will be charged at the end of each month.
To avoid margin call and liquidation, you must maintain a Minimum Balance, as well as limits on your account’s overall leverage via the Maintenance Margin Ratio.