Say a mass of $35kg$ is placed in the middle of a trampoline and causes a vertical displacement of $22cm$. Why is the spring constant $35g/0.22$? Why are we considering the applied force rather than the force within the material itself? Why does that work? It certainly doesn't work in the case of a mass being a hung on a horizontal rope, so what logic is one meant to use in order to deduce that the spring constant is $35g/0.22$?
Here's where the question comes from, for your interest: https://qualifications.pearson.com/content/dam/pdf/International%20Advanced%20Level/Physics/2013/Exam%20materials/WPH05_01_que_20150618.pdf