There is no separation between cause and effect. He has willed what you are, and you share His will with Him. You can therefore will to extend the Kingdom of God and to Create in His likeness.
However, the idea of a separate will is an ego idea. It's really the attempt to STOP the will of God, which means to not have a will at all. It is wishful thinking, because it can't be possible to truly have a separate will.
If you will with God, you will have Life. If you will against God, to have a different will, you will have death. Death is the absence of real will.
Also when you will against God, which is really a withdrawal from willingness, a refusal to will, you will go into the ego split mind in which there will appear to be "your will" and an external "alien will". You'll also believe the body has its own will along with everyone else. Multiple split wills.
Once cause and effect are separated out it's going to appear that everything external has its own will, and that it is willing against you, because you no longer see the connection between cause and effect. So you can't have your "own" will without simultaneously creating an "alien will" that is in opposition to your own. Therefore your separate will is the persona, and the alien will appears to be the shadow. And then everything "you" will separately will be thwarted by the shadow, and you'll be at war - a battle of wills.
As the Course says, there is a metaphysical curriculum of awakening with fundamental truths you need to come to. You cannot change that curriculum because it's the same for everyone (even outside of ACIM), but you can seem to delay learning the truth (the time you take it). You do that by seemingly having your own ego will which goes off busying itself and spinning its wheels and avoiding and denying, which you will then think is free will.
You are free to choose because God's love is not forceful and won't stop you from trying to be separate, but when you are without God's will you will have barely any will at all. That's why you can't perform miracles, which are supposed to be happening all the time. Without God's will miracles are impossible, and Life is impossible.
You will think you have a "life", and that it's your own life, which you control, or have autonomy over, with an authority separate from God, but that's an ego claim. Its a delusion, because in truth your will and God's will are shared, and will only exists when there IS a sharing of real will (God's Will). Will has to be extended, and the ego's will does not extend, it just projects.
Also the ego does not really ever will that you wake up or accept truth or be open to sharing with God or to receive a "cure" that undoes the ego.. It "wills" that you be dead, which is a giving up of the "will" to live. All attempts to have a will separate from God are attempts to relinquish free will and be a victim - the victim who appears to have no will power to actually take action or prevent death. And then the death of the body-identified person is allegedly the death of God's will. The end of Eternal Life.
In a similar vein to that you think you think, but you really only share the thoughts of the thought system you are aligned with, you "think you will", or wish you will, according to the thought system you belong to. Either you are willing "with" the ego, or you are willing with God. And the ego's "will" will be against God's will, because it's just a device for denial. Then whatever the ego wills, which is your death, will become what you're actually choosing and will experience. If you don't want that, you have to align with God's will and accept eternal life. You don't have any other options outside of which of the two thought systems you share in.
You can be of service to God, or of service to the ego. There is no other "separate" will that you have of your 'own' independent of both. Either you're in the "Life Will" or in the "Death Wish".
God's will not my will be done.