Provided the type of research you have added supports adding a DOI, you can generate one by following these steps:
Go to your Research tab
Select the research item you would like to generate a DOI for by clicking on its title
On the right-hand side, click on the blue downward-facing arrow and select Generate a DOI (if this is not visible, then generating a DOI for this type of research item is not possible)
Review the details of your research item to ensure they are correct
Click Generate a DOI.
Note: Once a ResearchGate DOI has been generated for a research item, you’re no longer able to edit that research item. Instead, you should remove the research completely, re-upload it with the edits and generate a new DOI.
Go to your Research tab. Select the research item you would like to generate a DOI for by clicking on its title. On the right-hand side, click on the blue downward-facing arrow and select Generate a DOI (if this is not visible, then generating a DOI for this type of research item is not possible)