In Administration - Register - Accounts you can specify default objects for the account.  However, this should, if possible, be specified on the account in the financial system and with integration being transferred to Palette.
The objects that are specified as default objects for the account in Default object for type will automatically be given as suggestions when doing the account posting. However, this can be changed by the user when posting.
You should also ensure that the Default object for type settings match the Use object of type settings. See below example. Objects of the type CostCenter should always be specified and the default object that will be proposed when posting is CostCenter 10.