Create a document property

from Spotfire.Dxp.Data import DataProperty
from Spotfire.Dxp.Data import DataType
from Spotfire.Dxp.Data import DataPropertyClass

propName = "docProp"
attr = DataProperty.DefaultAttributes

prop = DataProperty.CreateCustomPrototype(propName, DataType.String, attr)

Document.Data.Properties.AddProperty(DataPropertyClass.Document, prop)
prop.Value = "test"

Thanks Sean!