In Mark 11:23-24: Jesus,
You told me, that if I speak to any mountainous thing that seems to rise up out
of nowhere to block my path and intimidate me, saying, "Get out of my way, and
be slam dunked into the deepest sea"; and you said that if I don't doubt in my
heart, but only believe in good faith that what I said will come to pass; I will
have exactly whatever I said.
You told me, that in the same way, whatever I desire, when I pray, if I believe in good faith that I receive them, then I will have them.
Most often folks focus on the first part of this scripture, but let's look at the rest of it.
"Whatever I desire, when I pray, if I believe in good faith that I receive them, then I will have them."
Now this especially works well, if the whatever you desire and are praying for is something that God has already offered you in the Bible. But, will this work for other things you desire too? The answer is Yes and No. I say that because human beings tend to want to run before they learn how to walk, which often leads to disappointment if not disaster. Unfortunately, due to this being exaggerated or incorrectly taught, some have come to believe that either "this faith stuff doesn't work" or that God is unfaithful.
God primarily operates with man through contractual relationships. And then, through the Bible, He has made available thousands of good faith offers for us to convert into those contracts with God. In other words, he was the author of all of the contracts offered to us.
So, people want to know, "Can we write our OWN contracts with God?" And the answer is yes, but with a qualifier: Be it with God or man, one generally needs to establish, develop and maintain a good reputation for successfully fulfilling contracts authored by others before he or she would be deemed reputable enough for anyone else to enter into a contractual relationships with them, including God.
You see, God KNOWS that He is faithful, trustworthy, and reputable and always says what He means and always does what He says. In fact He says, "I am not a liar, like man is." God KNOWS that He can be trusted, but He needs continuous reassurance in His contractual relationships with us. This is why praising and thanking God is such an important part of receiving what you are believing for. It's called using the shield of faith. Praising and thanking God for whatever you are believing for, even before it manifests in the natural, is how to reassure God by using the shield of faith.
If you have a long, successful track record or reputation of fulfilled contractual relationships with God, then I see no major obstacle with writing your own contract. However, understand that since God was not its author, He is under no obligation to sign it, just because you wrote it.