test.cs

// create a new SubTextPart with the name "part1" and id 1
TXTextControl.SubTextPart subTextPart = new TXTextControl.SubTextPart("part1", 1);

// create a new UserInfo object
UserInfo userInfo = new UserInfo()
{
    Name = "John Doe",
    Address = "123 Main Street",
    City = "Dallas",
    State = "TX",
    Zip = "75201",
    Phone = "214-555-1212"
};

// serialize the UserInfo object to a JSON string
subTextPart.Data = JsonSerializer.Serialize(userInfo);

// add the subtext part to the SubTextParts collection
textControl1.SubTextParts.Add(subTextPart);