In short, the way you described, there’s no way to do that.
They are two distinct systems, without web service or any type of authentication together, except the same users and passwords.
Only with these factors it is not possible to do what you want, even for safety reasons.
If there was a Web Service for authentication, or other mechanism, such as Oauth from Google, that might be possible.
Possible attempt
An attempt to do this is to develop a Web Crawler to fill in the data entered in your system (after authentication) on the login screen of the external system. However, this can be relative in the type of authentication used in the other system.
Another attempt is to develop a functionality on the external system to receive the authentication data (preferably encrypted) and perform the authentication on the other system. But this would require modification in the external system, as there should be no such functionality.
Let me get this straight. Do you want the user to authenticate on your system and automatically authenticate on another that you don’t have access to sources? That’s what you’re looking for?
– Randrade
exactly. I can’t change the external system
– Gustavo Correia
you need to interact with it through a service (an API, a Webservice...) and that other system has to provide this service so that you can consume...
– Mateus Mattos
The external system has a correct login screen?
– durtto
-1 for requesting a tutorial.
– durtto
@Randrade technically there is no right answer? Is that illegal? If a system that does not provide alternative means of authentication, trying otherwise would not be illegal?
– durtto
no, my system needs to communicate with a client-used Helpdesk system.
– Gustavo Correia
Yes, the external system has a login screen
– Gustavo Correia
@durtto Illegal is not because it owns the user and password. Illegal would be if it "forced" the password break, or something invasive.
– Randrade