When coherent light is coupled into a multimode fiber, it forms a complex speckle pattern at the output, which results from interference between the different fiber modes. It was recently shown that by shaping the input wavefront, the light at the output can be focused to one spot. Such focusing can be used to achieve imaging through the fiber. There are various approaches to determine the required input to form a focus, all of which require access to both ends of the fiber. We show that by using non-linear two-photon fluorescence as feedback, it is possible to focus light at the output of a fiber, with access only to the fiber input. We further demonstrate an ability to control the focus position, a key requirement for developing a multimode fiber based endoscope.