how can i calculate R-squared for lsqnonlin function ?

i want to calculate R-squared of non linear regression of exponential function. but i don;t know what is its code??

the code that i have written:

clear;

clc;

close all;

%mix code: S100+NaOH 11.25 molar _conc_T8

%tdata = [0.5694 1.0000 2.0000 3.7326 8.0000 16.0000 32.0000 64.0000 ];

%sdata = [1.4367 1.1567 1.3750 2.9900 3.2000 33.0050 0.0000 46.3050 ];

tdata = ...

[0.5694 1.0000 2.0000 3.7326 16.0000 64.0000 ];

sdata = ...

[1.4367 1.1567 1.3750 2.9900 33.0050 46.3050 ];

beta= [1.003908069]

fun = @(x)x(1)*exp(-(x(2)./tdata).^beta)-sdata;

x0=[40,0.9];

options = optimoptions(@lsqnonlin,'Algorithm','trust-region-reflective');

[x,resnorm, residual] = lsqnonlin(fun,x0)

plot(tdata,sdata,'ko')

hold on

syms tlist

y = x(1)*exp(-(x(2)./tlist).^beta);

tlist = linspace(tdata(1),tdata(end));

y = subs(y);

plot(tlist,y,'b-')

xlabel time(day)

ylabel strength(MPa)

title('exponential Fit to Data')

legend('Data','exponential Fit S100 11.25 M conc T8')

hold off

[Abdolkarim Mohammadi]

https://blog.minitab.com/blog/adventures-in-statistics-2/why-is-there-no-r-squared-for-nonlinear-regression