Using integral in a fit function

I am new to root and I have been trying to do a fit using Birks’ function which contains an integral. I know how to define my own function for the fit but I don’t know how to include the integral in it. If someone knows a simple example to understand or a solution I would really appreciate it. Thank you in advance. Note that the parameters are the S and kB circled in blue.

_ROOT Version:6.24/06
_Platform:Ubuntu 20.04

See the example posted in Fitting in ROOT with a function having two integrals in it - #10 by moneta