To enter the prime symbol, you can click on the ' button located on standard keyboards or on the Desmos keyboard under FUNCTIONS and Misc.
To use prime notation for derivatives, first try defining a function using f(x) notation. f'(x) can then be used to graph the first order derivative of f(x). Use f''(x) to find the second derivative and so on. Note that depending on the complexity of f(x), higher order derivatives may be slow or non-existent to graph. If the derivative evaluates to a constant, the number is shown in the expression list instead of on the graph. In some cases, you may want to use d/dx notation for derivatives.