If you are using a Customer Lookup then you will usually see an account and a contact field in the list for the fields schemaname, select the contact one (which you may have done if its the correct schemaname for the field in question)
The metadata setting you have for Contact should be working on Save, if you need to show it on load above the Set Value On Save setting is the Prepopulate Field section.
Note: you have a form specified for the Lookup on that metadata record - not sure if you want it or not - this is the form that it will try to use to allow the portal user to create a new Contact record (as that is the table the Lookup is for), if the Contact will always be the portal user then clear the field.
To populate Case on the create form/table
On the Basic Form for the Create, under the Associated Table tab, set "Set Table Reference On Save" to Yes, this will then unhide fields to fill in for the Save (the Table etc will usually be the Table your main form that is displaying the subgrid is for .eg. case) it will also unhide a Checkbox for Prepopulate Lookup Field - check this prepopulate if you need to see the field displayed with a value on your create form (if you don't need to see it you can leave it unchecked).
