What is high stress? Soil mechanics books will help you judge this. What is excessive deflection and what criteria should I meet? It is project specific and needs experience to judge based on specific site conditions. You can see some interesting things happen like the face of excavation actually bends backwards or soil may bulge upwards! Try to go the other way and put very high prestress value, like put another zero or two zeros next to the values in the example. The deflection is smaller as well, stresses are more under control.

Now Try to setup the initial values some number higher than zeros and run it again, the difference between initial value you input as prestress force and final value the model calculates in smaller for both anchors than the first trial. You will find that the load in the anchor increases from zero by a large margin to its final load. or some large tensile stresses behind the face or where near ground surface where confinement is not that high. Lets assume this is an elastic FEM analysis and If, lets say, you set the prestress in anchors to 0 and run the model, you will find large deflection in the system and shear stresses in excess of shear strength of the soil at some locations, maybe. You will need to run the model a lot of times to find the optimum prestress values that will achieve your performance criteria - namely the stresses in soil and structural member are within acceptable limits but more importantly the deflection criteria is being met.

The initial value of the anchor prestress force can be any value. And absolutely you should not ask structural engineer to give you anchor load. You should not mix finite element analysis and limit equilibrium together.

Plaxis 2d anchor