See "How do I confirm authorship of my publications to add them to my profile?" in https://help.researchgate.net/hc/en-us/articles/14293005132305-How-to-add-research for instructions.

Go to the publication page by clicking on the publication's title. Click the More button at the top right of the page and select Claim authorship - is this your work? from the drop-down list. Select the author name you published under, or add if it's not yet appearing.