Steps:

• Login as an Admin account > Jira administration > Issues > Issue linking
• Add New Link Type:

 {{Name: Name
Outward Link Description: Outward </option><script>alert(1)</script>}}
Inward Link Description: Inward <script>alert(2)</script>

• Login to other Administrator (Higher privileges) > Jira administration > Manage apps > Issue Type Mapping > Untick Sub-Tasks/Links > Issue Link Type => Xss will be trigged