How to map a lookup field??

Hi All,

I want to map customerid (OOTB lookup field on the case) to a custom lookup field on a custom entity.

I have created a 1:N relationship between CASE and SAMPLE (custom entity) and added the field to the mappings:

I have a subgrid on the CASE form displaying all the SAMPLES relating to that CASE - so when I create a new SAMPLE using the "+" sign on the subgrid I would expect the customerid to be pulled through to the field on the sample???

Thanks in Advance.