You should consult the official website for details https://www.doi.org/ It can be generated by organizations and Journals and not by individuals

When you upload a research item including an article on ResearchGate without a DOI, the system asks you to generate a DOI.

Dear Ajay Kumar Farooq Azam Rathore , On the research RG platform, DOIs can be generated only for unpublished work (i.e. Preprint and Thesis) and published items such as article, book, chapter, patent, cover page, poster, and conference paper are considered to have been published elsewhere, DOIs cannot be generated for these types of research.

Ajay - ResearchGate is a forum that promotes research - but it does not 'generate' research. It is not their role to set up systems - such as DOI referencing. One should also be wary about journals that do not 'automatically' apply for DOI registration.

Thank You for the correction Nand Lal Kushwaha

DOI are tied to CrossRef...

https://www.crossref.org/education/why-crossref/the-problem-crossref-is-here-to-solve/

This link might also be useful: https://www.crossref.org/reporting/members-with-open-references/

Thanks a lot everyone for your responses

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you cannot add the doi to your article only the journal can it.