To apply for the revocation of the cancellation of their registration under Rule 23(1) of the GST Act, a registered person should use FORM GST REG-21.

Explanation:
1. Introduction:
- Under the GST Act, if a person's registration is cancelled by the tax authorities, they have the option to apply for the revocation of the cancellation.
- Rule 23(1) of the GST Act governs the procedure for such revocation.

2. FORM GST REG-21:
- FORM GST REG-21 is the form that needs to be filled and submitted by a registered person to apply for the revocation of the cancellation of their GST registration.
- It is titled "Application for revocation of cancellation of registration."
- This form is available on the official GST portal and can be accessed by registered taxpayers.

3. Procedure for filling FORM GST REG-21:
- The registered person should log in to the GST portal using their credentials.
- Navigate to the "Services" tab and select "Registration" from the drop-down menu.
- Click on the "Application for Revocation of Cancellation of Registration" option.
- Fill in the required details in the form, which include the GSTIN (Goods and Services Tax Identification Number), legal name, reason for cancellation, etc.
- The applicant must also provide supporting documents, such as bank statements, financial statements, etc., to substantiate their claim for revocation.
- After filling in all the details, the form should be submitted electronically.

4. Acknowledgement and processing:
- Once the form is submitted, an acknowledgement number will be generated.
- The tax authorities will review the application and the supporting documents provided.
- If the authorities find the application and documents to be satisfactory, they may approve the revocation request.
- However, if any discrepancies or issues are identified, the authorities may reject the application and inform the applicant accordingly.

In conclusion, FORM GST REG-21 is the appropriate form that a registered person should use to apply for the revocation of the cancellation of their registration under Rule 23(1) of the GST Act. It is essential to provide accurate information and supporting documents to increase the chances of a successful revocation.