@Override
	public boolean initialize(CodeUnit fromCu, int fromOpIndex, int fromSubIndex) {
		isValidState = false;
		this.editRef = null;
		this.fromCodeUnit = fromCu;

		Program program = fromCu.getProgram();

		populateExternalNames();
		extLibName.setSelectedItem(Library.UNKNOWN);
		extProgNameChanged();

		extLibPath.setText(null);

		extLabel.setText(null);
		extAddr.setProgram(program);
		extAddr.clear();

		extLibName.requestFocus();

		return setOpIndex(fromOpIndex);
	}
Pop up the data tree dialog so the user can choose the external program.
Ghidra/Features/Base/src/main/java/ghidra/app/plugin/core/references/EditExternalReferencePanel.java