If you have a valid UK VAT Number and EORI you do not need to specifically apply or register to use Postponed VAT accounting.
If you have a Customs Agent dealing with your declarations, you must ensure that they include your VAT Number and EORI on your import declarations.
You will need to ensure that you provide your monthly postponed import VAT statement (MPIVS) to your Spondoo bookkeeper to ensure these are accounted for in your VAT returns.