If I purchased a vehicle that I use in my S-Corp business, how do I enter it as an expense for my self employed income?

I strongly suggest you seek out a tax pro. you don't know the difference between an S-Corp for which a separate tax return had to be filed by 3/15/2021 (now 3 months late with a late file penalty of about $600 per shareholder assuming you file by 6/15)  and self-employment income.   it is unlikely you would file correct returns which could lead to additional penalties and interest.   

@Mike9241 See OP's other posts.