To apply for a postal vote for a referendum in the UK, follow these steps:

1. You must be registered to vote before applying for a postal vote.
2. You can apply online through the UK government website, or by completing and sending a postal vote application form to your local electoral registration office.
3. When applying, you will need:
   • The address where you are registered to vote.
   • Your National Insurance number or other identity documents (e.g., passport).
   • The specific date of the referendum if you want a postal vote for that one event only.
4. If applying online, you may also need to upload a photo of your handwritten signature on plain white paper.
5. You must submit your application by 5pm at least 11 working days before the referendum date.
6. If you live abroad, apply as early as possible since postal delivery can take longer.
7. If you decide not to vote by post anymore, notify your electoral registration office by 5pm, 11 working days before the referendum.

You can download the postal vote application form from the official electoral commission or government websites or request it from your local electoral registration office. Send the completed form to your local electoral registration office. This process applies generally for elections and referendums in the UK as provided by official sources.