After clicking next, you will be presented with a new window, where you will be asked to enter the amount of free space you want to add. In my case, I am adding 20GB to the E drive which has size 78.13GB making it 98.13GB. Add the size you want and click next.
When you click on next, you will be presented with a window of the summary of the operation. Just click on “proceed”. After you have clicked on “proceed”, a window will pop up giving you the time it will take for the operation to complete, click on “Yes”. The operation will then start, you wait for a while depending on the size you want to add: larger size requires more time, and the vice versa.