Hydraulic mtb-style brake lever on drop handlebars
Shimano's new GRX line include "sub-levers" that seem to fit your needs.

They are meant to be mounted on the tops of drop bars near the stem and run along the same hydraulic line as your main hydraulic lever/shifter. The product ID I believe you need would be BL-RX812-R (or -L if the left was desired)



https://bike.shimano.com/en-EU/product/component/grx-11-speed/BL-RX812-R.html