How to get part of the call method on Laravel

I have to many relations in my API where the user can pass on the URL like this &relation&relation2&relation3, in my controller I test if this relation is been called in the URL and call a method like withRelation, withRelation2, etc..

How can I create a main method called with($relation) and if I call withRelation it transforms into with('relation')?

Actual Examples:

withProducts should transform into with('products')

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