Unfortunately, it is not possible to add citations manually. See also these discussions:

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

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

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

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

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

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

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_add_missing_citation

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