Former President Donald J. Trump told a group of oil executives and lobbyists gathered at a dinner at his Mar-a-Lago resort last month that they should donate $1 billion to his presidential campaign because, if elected, he would roll back environmental rules that he said hampered their industry, according to two people who were there. About 20 people attended an April 11 event billed as an “energy round table” at Mr. Trump’s private club, according to those people, who asked not to be...