Escrow domain name sales are handled via escrow.com. If you do not have an escrow.com account, first create one, then go to Account Settings>Escrow.com, update your escrow account information, and click 'Save'.
Next, go to the Manage Domains page, select your domain name(s), click the 'Domain Settings' button located above the domain table, set 'Sale Status' option to 'Escrow', and click 'Save'.