1 Kings 1:34
And let Zadok the priest and Nathan the prophet anoint him there king over Israel: and blow ye with the trumpet, and say, God save king Solomon. And let Zadok the priest and Nathan the prophet anoint him there king over Israel: and blow ye with the trumpet, and say, God save king Solomon.
Zadok the priest and Nathan the prophet anointed Solomon king over Israel, blew the trumpet, and proclaimed him king. David directed this to be done at Gihon. Solomon was to succeed David in accordance with God's choice and direction. The anointing and public announcement carried out David's decisive appointment of Solomon as his successor.
David then promptly abdicated in favor of Solomon. Their prompt action thwarted Adonijah's attempt to seize the throne, thereby crushing the conspiracy and securing the kingdom.