https://www.tax.service.gov.uk/self-employment-support/enter-unique-taxpayer-reference and then enter your Unique Tax Reference Number and National Insurance Number.
You should then see the following screen:-

This will tell you the date and time from which you can make your claim. Click continue, and you will see the following screen:-

If you do not have a Government Gateway User ID, click on the blue link that says create them now.
We are aware that some applicants are getting the following message:-

Please note that in some cases, this is NOT correct. If you believe that you are eligible to make a claim, please click on the above link in blue that says – ask HMRC to review your eligibility
Please also note that unfortunately Accountants cannot make this claim on your behalf. Good luck with making your claim.