We would like to have the ability for our Sales Reps to be able to create a new case from within an opportunity screen.  That case then would be related to that opportunity only.

Salesforce allows you to see related cases once you enable the relational list for cases in the layout edit mode, but does not give you a NEW CASE button anywhere in the Opportunity screen.

Here is what I have done so far. 

Under CASES > FIELDS I created a custom relationship lookup field called RELATED OPPORTUNITY | API Name: Related_Opportunity__c.  I added the new lookup field to the case layout.  It works fine from within the case screen.  I can lookup opportunities with no problem.  Under CASES > BUTTONS AND LINKS I created a new list button called NEW CASE.

Button Details:  

/500/e?retURL=%2F500%2Fo

&cas4_lkid={!Account.Id}

&cas4={!Account.Name}

&006M0000004yR7w_lkid={!Opportunity.Id}

&CF006M0000004yR7w={!Opportunity.Name}

&opp4={!Case.Account}

&CF00N00000007E8N3={!Case.CaseNumber}

&cas3_lkid={! NULLVALUE (Case.ContactId, Contact.Id )}

&cas3={! NULLVALUE (Case.Contact, Contact.Name )}

&cas6=Pre-Sale

&cas11=AE - Web

&retURL=/{!Opportunity.Id}

&saveURL=/{!Opportunity.Id}

When I hit the NEW CASE button within the opportunity screen, it takes me to new case creation screen like it should.  Some fields like Account Name gets updated with the correct info except for the RELATED OPPORTUNITY field.

Where am I doing wrong?  The ID of the RELATED OPPORTUNITY field is: 006M0000004yR7w

Please help.