Service desk customer login
To vote, comment on roadmap portal cards, one needs to login on the portal. You can click on the login link at the top right corner. You will be navigated to JSD customer portal login screen. Login there & you will be brought back to roadmap portal in logged in state.
Service desk agent login
As a service desk agent, you must already have login details to your Jira service desk
You must be added as team member in the relevant service desk project
If you are not added as member, you won't be able to see any details on the roadmap portal. In such a case, get in touch with your JSD project administrator and ask them to add you as team member in the project. After that you should be able to see roadmap boards on the relevant JSD project.