Add new fields to WorkItem when migrating.

[teffs1]

So this question has been probably asked a million times but how do I migrate custom fields to work items.
I've tried this in the configuration
"FieldMaps": [
{
"$type": "FieldValueMapConfig",
"WorkItemTypeName": "Epic",
"sourceField": "Custom.InitialRequirement",
"targetField": "Custom.InitialRequirement",
"defaultValue": ""
}
],
But I get this message:

Microsoft.TeamFoundation.WorkItemTracking.Client.FieldDefinitionNotExistException: TF26027: A field definition Custom.InitialRequirement in the work item type definition file does not exist. Add a definition for this field or remove the reference to the field and try again.

[MrHinsh]

If the field exists on both the target and the source with the same refname then it will just migrate.

In this case it looks like the field Custom.InitialRequirement does not exist on the target.