var ifcItemUserPropertyService = new IfcItemUserPropertyService(serviceConfiguration, projectUrl);
// to get one user proper info knowing the key
var udp = await ifcItemUserPropertyService.GetAsync("up1");

//to create a new user property
 var newUdp = new BpNewIfcItemUserProperty
 {
    Name = "Project Start Date",
    Active = true,
    Category = "global", // the default Category of user property; this value depends on your permissions
    SubCategory = "Dates", // free text
    ValueType = "date",
    Index = 10 // display order in UI
 };
var createdUpd = await ifcItemUserPropertyService.CreateAsync(newUdp);