There is no automatic pop-up in ServiceDesk.
You have to use the 'Copy related attributes' functionality.
It ables you to copy an attributes value to another (from Category/Description to Incident/Description for example).
You can look at the LDSDDesigner documentation (Windows Manager / Copy related attributes).
If you need help, please ask me.
I too am trying to have a window associated action on a process show a window automatically. I tried to do this by putting the window associated action on the process as an automatic action but it doesn't work the way I'd like it to. The only way I can think of having it work is by putting a pre-condition that checks that the category is a specific category and then having a manual action available to add the extra info you need if the pre-condition is met.
This im sure is far from what you want but Im basically trying to do something very similar and trying to figure out a way myself.
You can have Service Desk automatically open a window for a manual action if you set in its properties the 'Is autoprompt action instance' to true. This will turn the line between the previous status and it red. As soon as the status is entered the window for this action will immediately pop-up, though it can be closed if the User doesn't want to fill it in and save.