Show auto number from customer entity on new created record on order entity

I have created new customer entity call PO which has an auto reference number which is a text field  and every time a new recorded is created on the PO entity a new incremental number is generated for the new record.  

I would like to show the last generated number which was create for PO rerecence number on new recode on CRM Order Entity?

is this possible?  I try to use a workflow however I cannot quires the PO Entity   

I would be grateful if someone can help.

Thanks,

PJ Moka