See this help page for instructions how to add published or unpublished research to your ResearchGate profile: https://help.researchgate.net/hc/en-us/articles/14293005132305-How-to-add-research

To upload a preprint, first, select a suitable preprint server or repository such as arXiv or OSF, and check their specific submission guidelines. Next, register or log in to the platform, then look for an "Upload" or "Submit" button to begin the process. Finally, fill in the required metadata (like title and authors), upload your manuscript file(s), and choose a Creative Commons license before submitting for publication.