If you mean ResearchGate by "website", so adding references manually is not possible. A method was proposed which may help a little bit, but only in special cases: https://www.researchgate.net/post/Method_for_Improving_Recognition_of_Citations_References_on_ResearchGate

For missing citations in general, see the following discussions:

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

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

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

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

https://www.researchgate.net/post/Why_are_some_of_my_citations_not_shown2

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

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

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

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

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

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

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

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

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

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

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

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

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

Which software are you using? There is option of editing reference in all the softwares where you can manually add the missing details.

(Does it depend on which website?) it may depend on the website.