In the cWebSessionManagerStandard (which you probably use) Ln56 -> Send Request_Save of hoSessionDD
You would need to fill phoSessionDD of your oSessionManager with your WebAppSession_DD-Object containing the right structure (Server set).

I recommend a subclass with Construct_Object having
Code:
Forward Send Construct_Object
Get phoSessionDD to hoSessionDD
Property Handle phoContCamType 0
Get Create (RefClass(cContCamType_DataDictionary)) to hoContCamTypeDD
Set DDO_Server of phoSessionDD to hoContCamTypeDD // or vice versa if your session table is the father.
Set phoContCamType to hoContCamTypeDD

This should set the structure.
Then saving should work because the DD is there.

P.S: I am using DF 18.1