You can decide to show the fitted line by using qqline = :fit.
But if you want to do it manually, you should build up each plot separately. So in principle:
p1 = qqplot(x,y)
plot!(p1, t1,0.0:1.0, label=string("Slope = ", round(t1.coeffs[2], digits=3)))
StatsPlots.plot(
p1,
qqplot(x,y))
This will not work though because t1 seems to define the wrong values.