...
You must be a Jira admin to add additional asset fields.
To
...
Create an Asset Custom
...
Field:
Go to Settings > Issues > Custom Fields.
Click Create custom field. The Select a Field Type popup appears.
Click Advanced > External assets platform.
Select Next.
Enter a name and optional description.
Select Create.
Select the screens you want this field to appear in.
Scroll down and click Update.
...