How to write html and javascript in custom field description?

[RSP]

Basically, I want to add help button to custom field  and on click of that button need to display some information. Prefer below image please as I want same.

[Nic Brough -Adaptavist-]

I would not try to do this, just present the full text to people instead.  Atlassian are doing the right thing in gradually removing the ability to inject dubious javascript into fields like that, so even if you could get it right, it's going to stop working soon.

[RSP]

But right now, working on this requirement.

please provide JavaScript code if you have any so that I can add in field description.

[Nic Brough -Adaptavist-]

I've not written bodges like this for years, since I realised they were such an awful thing to do and needed to stop.  I'm afraid I have no code to share, I've moved to doing things like this in different ways (ones that work properly and don't need maintenance)

I'd question the "need" for it first.  "I need to" is not a good reason to do anything, what is the underlying need for it?  What problem are you trying to fix?