Consider correlation matrix of fitting parameters in TF1 before a fit

I was wondering if it’s possible to include a known correlation matrix for certain fitting parameters before doing the fit. I have been reading other questions in the forum and read the documentation and I found how to get that matrix after the fit, but not how to implement it before the fit

We do not support directly in ROOT fitting a Graph (i.e. a set of points) which are correlated and you need to specify the correlation matrix.
In this case you need to create your self the least-square function using the given correlation matrix and use the Fitter class. An example on how to use the Fitter class for unsigned a user provided objective function (chi2) is this tutorial, line3dfit.C
If you need further help in writing the chi2 function, please let me know

Best regards