If the two fields don't have the same name and same case, then Tableau will not automatically recognize them as related. Therefore, a custom relationship will need to be created.
Hope this helps!
I am having this same issue trying to link up Teradata and Excel files. The common link between them is a date field which Tableau didn't automatically recognize because the names were different. I used the link that Tracy recommended to add a relationship, but Tableau didn't give me the red chain link icon. I'm not entirely sure what I'm missing...