'How to add significance markers (*) when comparing means and confidence interval in R

I need to compare the mean of each yeast with the reference(Rf) mean at a 90% confidence interval. I have plotted the graph below with the following code. The red crosses are the means.

ggplot(ANOVA_POST, aes(x=Yeasts, y=POF, fill=Yeasts)) +
  geom_boxplot(alpha=0.1) +
  stat_summary(fun=mean, geom="point", shape="+", size=6, color="red", fill="red") +
  theme(legend.position="none") +
  scale_fill_brewer(palette="Set1")

output

However, I was not able to place the "*" for the significance in the right places, as displayed. How could I do that, please?

I've tried also the steps from this post without success (Multiple comparisons with geom_signif function, R)

r plot mean confidence-interval

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'How to add significance markers (*) when comparing means and confidence interval in R

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