Isaiah recorded his experience in receiving his prophetic call: "In the year that king Uzziah died, I saw also the Lord sitting upon a throne, high and lifted up, and his train filled the temple." Can you envision that? Does it remind you of Lehi’s vision in 1 Nephi 1:8, where he saw "God sitting upon his throne, surrounded with numberless concourses of angels [the train that filled the temple] in the attitude of singing and praising their God."
Isaiah was in the temple, in the Holy of Holies, and he saw the Lord. If Isaiah saw the Lord, there must have been some reason for the Lord to come to Isaiah. The Lord does not make cameo appearances. We do not know what he told Isaiah. In the First Vision of Joseph Smith, Joseph was told many things. We only know a few of them. The Lord may have explained to Isaiah the basic Plan of Salvation, his mission, what the Lord would do, and how it would be part of the ministry of Isaiah to declare at least part of this message to others.
Book of Mormon Central, "How Did God Call His Prophets in Ancient Times? (1 Nephi 15:8)," KnoWhy 17 (January 22, 2016).
Stephen D. Ricks, "Heavenly Visions and Prophetic Calls in Isaiah 6 (2 Nephi 16), the Book of Mormon, and the Revelation of John," in Isaiah in the Book of Mormon, ed. Donald W. Parry and John W. Welch (Provo, Utah: FARMS, 1998), 171–190.