If you already have a DOI: See "How do I edit my research item's details?" in https://help.researchgate.net/hc/en-us/articles/14293081125777-Reviewing-editing-and-featuring-your-research for instructions how to add it to ResearchGate.
If not - a DOI is not compulsary, see https://www.researchgate.net/post/Is_DOI_compulsory_for_Published_paper.
If you wish to create a DOI anyway, see the two replies to: https://www.researchgate.net/post/How_can_i_generate_a_doi