If you want your JIRA fields do whatever you want whenever you want, you have to make a Live Fields Configuration and associate it with a project.
You can do this from Administration -> Add-ons - >Live Fields.
Info
For more information see Live Fields.
Add configuration
To add a configuration you have to click the Add Live Fields button.
In the displayed dialog box you have to enter the configuration name and description and you have to choose a SIL File for the Live Fields Configuration. Also you need to select some projects or some project categories to associate with the configuration.
where:
Name - Live Fields configuration name
Description - Live Fields configuration description
Script - the SIL script that will be executed on every issue page for the associated projects.
Projects - the associated projects
Project Categories - the associated project categories
You can edit and remove a configuration by clicking the Edit/Delete icon.