Currently we have a Login Portal App which able to jump into JIRA Service Desk. 
However we want use their credential that are saved in our portal to login into JIRA Service Desk. 

Problem
We have created a custom plugin which used REST API to handle creating new user in our portal and automatically create a user in JIRA Software. But the problem is, we are not able to set users password for JIRA. I know that JIRA handled all about user's password, but is there anything else that you can suggest for us?
We don't want to let the user interact with JIRA Authentication (Setting Their Password, Login by Username and Password).
We just want them to be able login to JIRA using their credentials that are in our portal. 

Thank you