To add a constituent code when promoting a non-constituent list record you will need to create a task set to include in a workflow to manage this.
Go to IOM>Configuration>Workflows>Task Sets>Add. Select the drop down in the first available row and choose "Promote to Full Constituent". In the same task set, select the drop down again and select "Add Constituent Code". You will need to open the "Configure Options" link that appears in the "Options" column to designate the appropriate constituency information. Save the set.
Go to IOM>List Management>Open List (whichever list is needed). Select the records to affect by highlighting the first selected record and using the CTRL key or the SHIFT key on your keyboard to choose multiple records at once. Right click on the selection of records and choose "Apply Task Set". You will have an option to select the created task set from the drop down menu that appears.