To work with Onestaff, you need to register as self-employed with HRMC.
This is an easy process which you can do here : https://www.gov.uk/register-for-self-assessment/self-employed Please note that you will need a National Insurance number in order to register with HMRC.

In order to register, first you will need to create a Government Gateway account. This grants you access to the government’s range of online services, the most important of which is HMRC’s Self Assessment portal. Your Unique Taxpayer Reference (UTR) number will be sent to you within 2 weeks after registration. You will need to submit this UTR number to Onestaff as soon as possible.

After that, you will need to give a name to your ‘business’, the most common is to use your own name.

Here are the steps to follow :
1 - Register online and follow the instructions to enter your personal details (make a note of your ID number and password, you will need it again at the end): https://www.access.service.gov.uk/registration/email

2 - Register for HMRC Taxes and choose ""I want to tell HMRC that I am in business and need to register for a new tax or tell HMRC that the company is now active""

Click on the ""next"" button until the page where you need to select one service to register and your type of business appears: choose Self Assessment for taxes and Self-employed/Sole trader/Sole proprietor for the type of business

3 - Complete all information ""About you"".

And then answer ""NO"" to all the questions on the next page.

4 - Fill in the information ""About the business"" :

Enter the date of the day you completed the form as the start date.
Specify that you work in the Hospitality or Catering field.

When you are beeing asked to fill in your ""Business address"", enter your home address and not Onestaff's

5 -  Declare that all your information are correct

6 -  You need to sign in with your User ID Number and password

You are done ! You will receive an email confirming your registration and in the following weeks a letter with your UTR number.