private String getLocationName() {
		String locationName = extLabelTextField.getText();
		if (locationName != null) {
			locationName = locationName.trim();
		}
		return locationName;
	}
Pop up the data tree dialog so the user can choose the external program.
Ghidra/Features/Base/src/main/java/ghidra/app/plugin/core/symboltree/EditExternalLocationPanel.java