How do I load constants in the curve fitting toolbox

Hi, I have data I need to fit a curve to. I have an equation for the curve that uses a load of constants which are all loaded in the Matlab workspace, along with a single constant which is what I'm trying to find. But when I use the curve-fitting toolbox, it treats all the existing constants as unknown parameters. How do I load these constants into the curve-fitting tool?

For example:

y = Mx + c

Where I already have c in the workspace.

[Sirisha Tadimalla]

In your fit definition, use (...,'independent',x,'dependent',y,'problem','c')