Unable to Authenticate custom connector with on-pr...

Hi Community, I was trying to get data from cisco DNA center which is only available inside my organization network. So I was using On-premise data gateway. How this authentication works is first we have to post request with username and password => then it will response with token=hsjsj. Then using that token I have to call main Api endpoints to get the data I want. So I wanted to achieve this however i am unable to get token. I am receiving authorization error. Please suggest me any workaround or solution. Thanks this is urgent i have a deadline to get the data and make dashboard in power bi.