workspace vector into cftool Custom Equation

Hello

I want to use cftool to fit an experimental dataset. Part of the fitting expression is a background measurement which should be scaled by some unknown amount, so the Custom Equation I would like to use looks something like this

A + B*BackgroundVector + MODEL

I have tried making BackgroundVector a function which loads the data and returns the vector but I always get an error that the matrix dimensions does not match. How can I load a workspace vector into an expression in cftool? I tested all functions outside the cftool and everything seems fine.

Thanks, Jacob