//load a sale by ID 
var sale = PublicAPI.CRM.Sale.Load(3);

//create a meeting 
var myMeeting = PublicAPI.CRM.Relationship.CreateMeeting(sale);

//myMeeting - created meeting for a lead 

//WARNING!! A meeting is created without a name. For correct work with the meeting, you should 
//specify its name and save the object to the database
myMeeting.Theme = "Meeting Name";
myMeeting.Save();