Custom script for including a custom field in the Quick entry dialog window while creating a new project

I have added a new field called “Assigned user” which I want to show in the Quick entry dialog window for creating a new project. This field needs to appear only when a certain Project type “Engine repair” is selected from the Project type drop down list. I have added this custom project type “Engine repair” already. I am getting partial results by making the custom field “Assigned user” mandatory. But the user needs to press on tab after selecting Project type as “Engine repair”. This works well for desktop but will not work in mobile devices or tablets. Hence what custom script do I write to make it work? Please note that the ERPNext is hosted on our own server.