Scopus is indexing selected journals automatically, and there is no way to add papers manually. The Scopus database of papers is rather incomplete, and my impression is that they neglect many journals that are not published by Elsevier - perhaps to urge authors to publish in Elsevier journals. See also https://www.researchgate.net/post/Does_anybody_know_how_we_can_add_the_missing_citations_to_our_profile_in_Scopus