Currently I am working on 3DX and JIRA integration. I wanted to save a JIRA URL in custom field without using any add-on or script which could be sent at 3dx end and JIRA issue link can reachable from 3dx end itself.
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without using any add-on / script means that you're only willing to use vanilla jira capabilities
sure you could store an url or whatever other string in a custom text field but that wouldn't do nothing towards making it readable by that 3DX program you mention.
I don't think jira is delivered out of the box including a working integration to 3DX (I don't even know what 3DX is, tbh...)
I'm pretty sure there must be a way to get what you want, there (almost) always is, but I'm also pretty sure it would make use of some add-on / script
