PAYE reference when paid under LEL

Hello, limited company director here trying to complete a self assessment tax return. Have been paid a salary within the LEL so no PAYE scheme set up for the company. However, when completing my tax return, I need to enter a PAYE reference, which I obviously do not have as there is no PAYE scheme set up. What do you enter here? The software doesn't let me leave this section blank

Hi
Please refer to the employment notes for SA  and the instructions for box 4, PAYE ref.
You may need to speak to the software provider
Box 4 PAYE tax reference of your employer