No End" Boundary Conditions in Wave on a String

[accdd]

How did they impose boundary conditions here if "no end" is selected?
Here: https://phet.colorado.edu/sims/html/wave-on-a-string/latest/wave-on-a-string_en.html
I would like to do the same thing without changing the wave equation of the string.

 

[Orodruin]

There is no wave traveling to the left. That fixes the wave going right if you fix the boundary to the left.

 

[accdd]

Thank you.
How do I eliminate the waves traveling to the left if I am simulating the wave equation with a simple finite difference method?