Hi, did you check Single Sign On - SSO? There are links to standard implementations, as well as an example to login programatically. 

Hello Anders, thank you for answering.

We managed to create the directory manager plugin so when our user open Joget portal they can enter their username and password from our current system to login.

However, we are not sure how to progress using the guide (in the same page as you mentioned). We are not using third party authentication provider.

Can you tell us the general steps that we need to make so that when user open Joget portal, they can either automatically login if they have already login on our system, or have a button to open our system login form to login? 

If the user is already logged into your system, you can try having it introduce a trusted token or cookie. In your Joget app, you can then check for the existence of the token, verify it and then perform the actual login using the code in the SSO code page.

We are guessing that the mechanism is as follow. Can you check if it is correct?

What API can we call to pass token to Joget?

Since it's your own token implementation, you should call your own plugin's web service to verify it before performing the programmatic login.