With Work in Progress you can provide feedback with Reactions and Comments. How you do this depends on where you are viewing Works in Progress.

On the web at Behance.net:

You can react to a Work in Progress by opening up a user's Work in Progress and hovering over the Appreciate icon. From here, you can click on the desired reaction you'd like to give. 

To provide more detailed feedback, you can you can click into Send Message on the bottom of the screen, then enter your feedback and hit Enter on your keyboard.  

Using the Behance iOS app:

You can react to a Work in Progress by opening a user’s Work in Progress and clicking the Appreciate icon on the bottom left corner of the screen. To select a different reaction, hold down the Appreciate icon and select your desired reaction. 

To provide more detailed feedback, you can you hit Send Message right on the bottom of the screen, then enter your feedback and hit Send.