Custom MetaData Types reference in Formula Field

Unfortunately, you can't reference custom metadata types in formulas at all. Their primary purpose is to provide metadata that's easy to package and transport between orgs, and primarily intended for use in Apex Code.

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You can now reference custom metadata types in validation rules. To do so, specify the object name, record name, and field value:

$CustomMetadata.MyCustomSetting__mdt.DeveloperName.SomeFieldValue__c

We are adding support for custom metadata type in formulas. In Spring '18 you will be able to reference CMT records in validation rules (pilot) with the future addition of formula fields, default values, etc.

https://releasenotes.docs.salesforce.com/en-us/spring18/release-notes/rn_forcecom_development_custom_metadata.htm

Thanks!

You can reference CMT records in formula fields starting Winter '19 release.

It follows the same syntax as in validation rules: $CustomMetadata.MyCustomSetting__mdt.DeveloperName.SomeFieldValue__c

http://releasenotes.docs.salesforce.com/en-us/winter19/release-notes/rn_forcecom_development_custom_metadata_types_ff.htm