get access token from credentials by Rest request

I'm trying to get the access token to attach to all the other request, what i want to know is how to call the authorization server for retrive a new Bearer token, In other words the equivalent of the following code using the rest api instead:

var autenticationContext = new AuthenticationContext(Config.my.PB_AutorityURL());
var credential = new UserPasswordCredential(Config.my.PB_Username(), Config.my.PB_Password());
var authResult = autenticationContext.AcquireTokenAsync(Config.my.PB_ResourceURL(), Config.my.PB_AppID(), credential).Result 

Thanks for your attention