Google dialogflow API Authorization

I need to integrate dialogflow project with oauth2 in my backend. please only experienced developers can bid and maximum budget 50$. it is small integration. Only payment using freelancer website is available. More details : The request will go from dialogflow to my server to request oauth2 authorization code then dialogflow will use this to get token and user at this point is authenticated. From Google assistant simulator it will synchronize with my server. Currently, the Google assistant simulator showing this message "account is not linked yet". Ftp account is available so you can make oauth2 accepted from Google assistant side.