Customizing "modern" lists and libraries new form

Hello Gayatri,

As of now, there is no functionality to customizing modern UI.

There are numerous customizations which currently are not supported for "modern" lists and libraries:

• JSLink based field customizations - See note on SharePoint Framework Extensions
• JSLink based view customizations - See note on SharePoint Framework Extensions
• Custom CSS via AlternateCSSUrl web property
• Custom JavaScript embedded via User Custom Actions - See note on SharePoint Framework Extensions
• Custom master pages - More extensive branding will be supported later using alternative options
• Customization via InfoPath
• Minimal Download Strategy (MDS)
• SharePoint Server Publishing

But Microsoft is working on following thing which is in SharePoint Framework roadmap

Client-side web parts++ and add-ins

• Supporting more complex scenarios and interactions with web parts

  • Part-to-part communication
  • JS Framework isolation
  • "citizen developer" model for lightweight development

• Bring add-ins to the modern world: Let’s make them play nicer with the new UX.

  • Azure AD Registration
  • Native responsive support
  • Build Add-Ins with SharePoint Framework

JavaScript embedding support (JSLink, User Custom Actions)

• The same tool chain and deployment model as client-side web parts
• Drive from a strongly typed base class wherever possible rather than manipulating the page DOM directly.
• Enable modern extension usage with modern experiences similar as Custom Actions and JS Link in classic experience
• Work with NoScript via tenant app catalog