Unfortunately, there is no way to add citations manually. You have to wait whether ResearchGate's automated algorithm finds this citation (or not). See also these discussions:

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

Make a list of the journals that have referenced your work. These citations can be located using academic search engines like Google Scholar or Web of Science. Verify again that the citations are correct and that your work is in fact being referred to.

the citation data in a suitable format. Include the author(s), publication title, journal or conference proceedings name, volume and issue number (if relevant), page numbers, and the year of publication for each citation. If available, you can also include the DOI or URL. Citations should be formatted using the correct citation style (APA, MLA, Chicago, etc.) for your field.