What do you mean you cant?
if you have Contacts::pkid, Contacts::fkCompanyid, and Company::pkID and they are corectly populated then just go to the relationship graph and click/drag a relationship between the pk and fk fields.
If you don't know how to do that then stop and get the FileMaker training series and work it.
When I was dragging it was creating a one to one. I removed auto-enter in foreign key and it allowed the one to many, but not behaving as I expected.
post your file and describe what you expect.
While there is a unique to FileMaker alternative approach, the general method for implementing a many to many relationship requires defining a third table that serves as the "join" table between the two tables that you want to link in a many to many relationship:
Company---<Company_Contact>-----Contacts (---< means "one to many" )
Company::__pkCompanyID = Company_Contact::_fkCompanyID
Contacts::__pkContactid = Company_Contact::_fkContactID
I just noticed that I misread the original post. You have asked for help setting up a one to many relationship and I described how to set up a many to many relationship.
Apologies for that.