Assuming you have 2 entities Boss and Company and an association between the company and the boss as * to 1.
This means a company has 1 boss and a boss can refer to multiple companies.
Create the overview and edit pages for both the entities. On the Company NewEdit page the boss can be selected with the reference selector, essentially assigning the boss to the company. Once this data is available you can add a datagrid inside the dataview of the boss NewEdit page and show the information of the companies by selecting the company over association. The on the boss page you’ll see the companies for that boss.
In Mendix FK’s aren’t used all associations are created in link-tables storing the key (id) from the left and right tables (entities).
Hope this helps in finding your solution