How do I link an evoucher to be set against by electricity usage to my OVO account?

Hi, earlier this year, I had an ECO4 grant, which has given me an air source heat pump, insulation and solar panels.

A couple of months ago, I was contacted by the company that did this work to say that I was eligible for evouchers - four times a year, for two years, to set against my energy bills.  The scheme that these are going to come through is - www.affordablewarmthsolutions.org.uk   (This is not the winter fuel payment)

So, I got my first evoucher today - £100 - and I’m not sure how to link it to my account, particularly bearing in mind that I pay by direct debit.

I went online to Ovo, spoke with an advisor who told me that they couldn’t help with this, but gave me a phone number to ring.  I’ve just come off the phone to a very pleasant woman from Ovo who also admitted that she doesn’t know how to do this.  

I suggested to her that I’d ask on the Ovo forum and she seemed to think that this was the best thing to do!!  

Can anyone help, please?

Many thanks, Cate