You can create a new DITA document and make it the target of an existing external xref.
Do the following:
To call up the
Change External Xref Dialog, choose or click the
Insert External Xref button
on the toolbar.
The Change External Xref Dialog will appear.
Figure 1. Change External Xref Dialog.

To call up the
Create New DITA Document Dialog, click the
Create New DITA Document button
.
The Create New DITA Document Dialog will appear.
Figure 2. Create New DITA Document Dialog.

In the Create New DITA Document Dialog, do the following:
From the Template drop-down list, select a template for the new document.
In the Title field, type the title of the new document.
The Id field gets filled in automatically when you are typing the title.
In the File field, specify the name of the new document.
The new document will be saved in the directory of the current document. To specify another location for saving the new document, use the Browse button.

The new document will be created.
In the Change External Xref Dialog, the newly-created document on the Files list and its top-level topic on the Targets list are automatically selected for insertion.

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You can also check the Close on Change checkbox, and then click the Change button.