By using a custom loss function that penalizes low predictions more than high predictions, you can coerce the network to make high predictions to a specified quantile value, such as 90th percentile.

Johannes Schmidt-Hieber, NONPARAMETRIC REGRESSION USING DEEP NEURAL NETWORKS WITH RELU ACTIVATION FUNCTION, The Annals of Statistics, Vol. 48, No. 4 (August 2020), pp. 1875-1897 ...