Insist on an answer

TI-BASIC, 2 bytes

:Prompt X

TI-BASIC does this automatically. Once input is given, it will quit.

Here is a GIF:

Watch the count on the enter key in the Key Press History. Created with TI-SmartView CE and ezgif.com.

Python 3, 18 bytes

while''==input():0

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JavaScript, 37 22 17 bytes

while(!prompt());

while(!prompt());

Explanation

The while keyword starts the while loop. In the condition of the loop, !prompt() asks for input and checks whether it is given or not. If it isn't given, the body of the loop is executed, which in our case is empty, then the interpreter goes back to the loop condition. The same process happens over and over again until the user gives the input.