As far as I know, there is no way to add citations to Google Scholar. In most cases GS is better than other platforms in identifying citations, but not always. Perhaps you have just to wait until GS's algorithm will complete the list. See also these related discussions (as you can see, this is one of the most frequenstly asked question):

https://www.researchgate.net/post/Why_citation_score_for_author_publications_looks_different_on_Scopus_research_gate_google_scholar

https://www.researchgate.net/post/Does_anybody_know_how_we_can_add_the_missing_citations_to_our_profile_in_Google_Scholar/1

https://www.researchgate.net/post/Is_there_any_to_add_the_missing_citations_in_Google_Scholar_Profile/1

https://www.researchgate.net/post/How-to-add-missing-citations-on-Google-Scholar/1

https://www.researchgate.net/post/How_can_i_add_missing_citation_to_my_Google_Scholar_profile/1

https://www.researchgate.net/post/The_article_published_in_a_scientific_journal_within_the_Scopes_container_Why_didnt_a_new_citation_appear_in_Google_Scholar/1

https://www.researchgate.net/post/Could_you_please_explain_my_inquiry_on_citations/1

https://www.researchgate.net/post/What_to_do_if_a_citation_is_not_included_in_Google_Scholar/1

https://www.researchgate.net/post/Why_some_article_citations_is_not_seen_in_the_google_scholar/1

https://www.researchgate.net/post/Do_you_know_why_some_citations_are_not_index_in_the_google_scholar_profile_but_the_same_citation_index_in_the_other_data_bases_such_as_scopus/1

https://www.researchgate.net/post/Why_are_some_citations_not_visible_on_ResearchGate_and_Google_Scholar

https://www.researchgate.net/post/In_Google_Scholar_my_master_thesis_got_a_citing_but_in_Researchgate_that_does_not_visible_Why_does_it_happen/1

https://www.researchgate.net/post/Can_someone_help_me_understand_why_there_a_difference_in_the_number_of_citations_shown_in_a_persons_Researchgate_and_Google_Scholar_account/1

https://www.researchgate.net/post/Why_is_citation_count_of_research_articles_different_in_Research_gate_from_that_of_google_scholar/1

https://www.researchgate.net/post/Why-does-my-article-citations-appear-in-google-scholar-but-not-included-in-research-gate/1

https://www.researchgate.net/post/Why-some-citations-are-not-recorded-which-are-found-in-google-scholar/1

https://www.researchgate.net/post/Which_is_more_accurate_in_reflecting_research_outputs_Google_Scholar_or_ResearchGate2/1

https://www.researchgate.net/post/Does_anybody_know_how_we_can_add_the_missing_citations_in_Research_Gate_as_will_as_Google_Scholar_profile/1

https://www.researchgate.net/post/Does_anyone_know_how_long_it_takes_for_Google_Scholar_to_pick_up_new_citations_from_ResearchGate/1

https://www.researchgate.net/post/Recent_publications_and_Citations_on_ResearchGate_are_not_getting_indexed_in_Google_Scholar/1

https://www.researchgate.net/post/Is_there_any_method_to_add_missing_citation_of_your_work_in_RG_As_most_of_the_papers_have_more_citations_as_per_google_scholer_but_very_low_in_RG/1

https://www.researchgate.net/post/How_to_include_all_citations_on_RG_and_Google_scholar_citing_your_manuscript/1

https://www.researchgate.net/post/How_can_we_add_citations_in_google_scholar_and_Research_Gate_for_our_articles/1

https://www.researchgate.net/post/How_can_I_link_my_articles_on_research_gate_to_google_scholar/1

https://www.researchgate.net/post/How_to_link_Research_Gate_article_citations_to_Google_Scholar_and_vice-versa/1

https://www.researchgate.net/post/I_have_4_citations_in_Google_scholar_can_these_citations_be_captured_by_RG/1

https://www.researchgate.net/post/How_to_add_missing_citations_on_Google_scholar_Research_gate_and_Scopus/1

https://www.researchgate.net/post/Does_any_know_how_can_I_add_missing_citations_to_google_scholar_and_resaerch_gate/1