How to get all lookups from a lookup field in Dynamics 365 CRM in C# Plugin

i made a little crm app that has the Entities : Contact, Campus, Sub Campus,
the contact has lookup fields for campus and sub campus.
(each sub campus is related to only one campus)
im supposed to make a plugin in C# that on contact creation it fills the campus field based on sub campus.
how do i get the campus through sub campus field...?