Cut that tall branch about halfway up your blue line. I am sure there is a healthy leaf on the outside of that branch (you don't want branches that angle towards the center of the plant). Use a pair of by-pass (not anvil-type) hand pruners, sterilize them with alcohol and cut 1/8" above that leaf at an angle so water runs off away from the bud/leaf you are choosing as your new terminal bud. This is a bonsai, it is important to keep roots and plant in proportion.
Later, there might be an even better leaf to choose that is a little lower. Don't go crazy as this cut is at least a 1/3 of the plant and it might get stressed a bit. Pruning on Bonsais must be done a little at a time.
Most of the energy in that branch is at tip. You are cutting off that energy expenditure and all that energy will be diverted to the rest of that branch to 'lateral buds' you can't see right now. This extra energy will tell that branch to wake up more lateral buds...they are there. Careful and educated pruning will make your plant healthier. Take a bonsai class! They depend on you for everything. One mistake and it'll die. Bonsai are the furthest thing from 'natural'...learning to care for a bonsai will teach you an awful lot and humble you. Plants are amazing lifeforms! Very different than animals!!