And Abraham said to God, “Oh that Ishmael might live before you!” (Genesis 17:18 ESV)
ויאמר אברהם אל־האלהים לו ישמעאל יחיה לפניך
The phrase אל־האלהים treats אל as a preposition: Abraham spoke "to" God (Elohim). However, it is obvious "to" whom Abraham is speaking and the preposition אל is unnecessary:
And Abraham said "HaElohim, Oh that Ishmael might live before you!”
ויאמר אברהם האלהים לו ישמעאל יחיה לפניך
The unmarked אל is also a noun meaning "God," which is is how God identifies Himself:
When Abram was ninety-nine years old the LORD appeared to Abram and said to him, “I am God Almighty; walk before me, and be blameless (17:1)
ויהי אברם בן־תשעים שנה ותשע שנים וירא יהוה אל־אברם ויאמר אליו אני־אל שדי התהלך לפני והיה תמים
Obviously, if Abraham had used אל־אברם then his request would have been understood as, "El Shaddai, Oh that Ishmael might live before you!”
What factors require אל to be read as the preposition and prevent Abraham from addressing God as El, HaElohim"
And Abraham said, "El, HaElohim, Oh that Ishmael might live before you!”