Check if the platform supports DOIs, Use a DOI registration agency like Crossref, Data cite and so on, provide meta data like title, authors and date, etc, ensure the DOI links to a stable location.
ResearchGate offers you a free DOI once you upload the poster to their platform.
To do this, please head over to the research items page on your profile. Then, choose which research item you'd like to generate the DOI for. Following that, you'll be presented with a page that has the details of the poster you uploaded. On the right-hand side, you may view an arrow pointing downwards and the word "More" next to the arrow. Click on that, and you will find an option to "Generate DOI".
There you go!
P.S. This was done on a PC. Options may differ if you are using a smartphone.