As suggested in below link, where to write javascript code?

[Prasanna]

Below given link suggest javascript code to be written. My question is where is the place where I can write this java script code in JIRA?

https://confluence.atlassian.com/jirakb/populate-a-field-based-on-another-field-s-content-in-jira-server-649921383.html

Thank you,

[Nic Brough -Adaptavist-]

The doc says "in the field descriptions"

[Adria Alonso]

Hello Prasanna,

We tend to write < script > < / script > tags in the announcement banner for simple scripts.

Very important TO HAVE ALL HTML TAGS CLOSED in the announcement banner.

For example I have a simple script that adds columsn to each tr of 

#ghx-issues-in-epic-table tr

Hope it helps