What´s the issue? Just set the wavelength on your CDS and a suitable reference, if needed. If you share more details about the model#, I can point you to the user manual.
Here´s an old, but useful document about DAD optimization:
For the reference wavelength, there´s a guideline on page 12 of the DAD optimization document. There is no need to use a reference, but often it´s beneficial. Make sure the reference WL is well separated from the signal WL, as shown in the document. There are many pages on the topic "reference".
I don't see why the tungsten lamp would be required at 400nm. You might have set a "Vis lamp required" somewhere in your CDS or one of your settings points to a higher WL.
Finding a suitable reference wavelength and reference bandwidth should be part of your method development. Please go over section "Sample and Reference Wavelength and Bandwidth" in the manual and see the pages for reference selection in the DAD document, I shared.
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