Tax and Tax code Query

I’m hoping there’s someone out there who can help. I’m currently working permanently for a company earning £30K before tax (PAYE). My previous employer has asked me to help them with a job which I’ve quoted at £3,750.

I’ve never done any sort of contract work on the side so would like to know how I declare it with the taxman. The company who I’m contracting for will pay me by cheque and for some reason they’re asking for my tax code to check I’m paying tax. Would I just fill in a self-assessment tax form next April? Also what would my tax code be?Thanks.