Create a new pop- up menu window
It depends on the place of UI where you want to create a form.
Below is link where you can find details about, how you can create action which will open form on click of particular document.
Adding new actions to the Document Library | Alfresco Documentation
I humbly suggest that you work through some of my tutorials, starting with the one on the SDK: Getting Started with the Alfresco Maven SDK | ECMArchitect | Alfresco Developer Tutorials
After that, you should work through the content types tutorial and then the one on UI Actions:
Creating Custom Actions in Alfresco | ECMArchitect | Alfresco Developer Tutorials
As Krutik says, depending on where you want the pop-up and form to occur, the UI Action is probably the most relevant to what you are trying to do.
Obviously you should also be looking at the Alfresco source code for similar functionality to what you are trying to achieve.
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