How can I add a custom field to a Blueprint in the New User Experience (UX)?

In order for custom attributes to be available in a blueprint, they need an API Attribute ID which is added in the Clarity Classic UI.

Steps to add an API Attribute ID to an attribute:

1. Log in to the Classic UI
2. Go to Administration->Objects->Project->Attributes
3. Click on the custom attribute
4. Enter an API Attribute ID value and save

To add the custom attribute to a Blueprint:

1. Go to the New UX and navigate to the Blueprints section
2. Click on a custom Blueprint. The custom attribute should now be visible in the Fields section of the Blueprint.
3. Once you add the custom attribute to the Blueprint, it will then show in the projects associated with that Blueprint. 

Exceptions

API Attribute IDs for one or more of the following attribute data types are not supported depending on your Clarity version, and thus these field types are not able to be added to Blueprints.

• Custom time-scaled value (TSV) 
• Attachment 
• URL

Reference In the New UX, Unable to add Custom Attributes to Blueprints      
Reference In the New UX, Some Attributes Cannot Be Added To Projects