computing a closed loop Transfer Function from an open Loop Plant.

Hello, I have been tasked in a project with taking an LTI model of an open loop plant that I previously created and generating two closed loop transfer functions from it with specific inputs and outputs (response y, reference r, and control u, reference r respectively). From there I need to use LSIM and apply experimental data for both inputs and generate a simulated response. I thought I could use "feedback(plant,1,feedin,feedout)"(1 because its a negative unity feedback) to close it and establish the inputs as r= feedin, and y= feedout, but that doesn't work. Is there a function in matlab that does what I need to do or should I try to analytically find the closed loop transfer function?