This is constant, whose numerical value is depending on initial conditions.
You can either specify it directly, per Solve system of differential equations - MATLAB dsolve (mathworks.com)
bnds = y(0) == -0.123
ySol(t) = dsolve(ode, bnds)
But you can also try to solve it substituting to general solution analytically:
Plotting and the example code
syms y(t)
ode = diff(y,t) == -sin(y)
bnds = y(0) == -0.123
ySol(t) = dsolve(ode,bnds)
tv = [0, 0.1, 0.7, 1.5, 3.0];
yv = ySol(tv)
figure,
plot(tv,yv,'o-')
