What position, other than president, does lincoln claim that gives him the power to issue the emancipation proclamation?

This Presidents and the Constitution e-lesson focuses on Abraham Lincoln and the Emancipation Proclamation. Though he had always hated slavery, President Lincoln did not believe the Constitution gave him the authority to bring it to an end—until it became necessary to free the slaves in order to save the Union. With the Emancipation Proclamation, which he viewed as an essential wartime measure to cripple the Confederacy’s ability to fight, Lincoln took the first step toward abolition of slavery in the United States

The congress completes the diagram