How do you mark an outlier on a box plot?

With a cross. This will be outside the lines that represent the maximum and minimum

With a cross to denote the outliers.

Outliers for box plots are defined by being more than 1.5*IQR (Interquartile Range) under the first quartile (Q1), or more than 1.5*IQR (Interquartile Range) over the third quartile (Q3). They are usually indicated as a cross or dot, outside of the box plot and unconnected to it.

Outliers are usually identified on a box plot by a cross.

You mark an outlier using an 'x' or a '+'. An outlier is defined as a data point that lies outside the whiskers of the box plot, usually taken to be outside 1.5 times the interquartile range above the upper quartile and below the lower quartile.

Plot the data, e.g., on Desmos. You may wish to watch this video, e.g., box plot on Desmos.