some ID field: e.g. CF00…. This corresponds to the ID of the field that you want to be prepopulated. This will always be the text that is displayed in the box.{some ID field}_lkid: e.g. CF00…_lkid. If the field in question is a lookup or master-detail, then this field should be set to the resulting record ID {some other ID field}. Note that in this case, the actual non _lkid field is irrelevant, and not consulted.
Answer for your question
/500/e?
CF00NC0000005Au31={!opportunity.name}&
CF00NC0000005Au31_lkid={!Opportunity.Id}
&RecordType=012C0000000MK5d
Should populate the opportunity in the lookup field
