I am a self-employed, where can I put Sep IRA for tax refund in 2018?

Enter self-employed retirement in the Search box located in the upper right of the program screen.  Click on the Magnifying Glass.  Click on Jump to self-employed retirement
Or-
Click on Tax Tools on the left side of the program screen. Click on Tools. Click on Topic Search. Type in self-employed retirement. With the key words highlighted, click on GO.

A SEP IRA contribution wont necessarily give you a refund, it just reduces income subject to income taxes...it wont reduce any self employment taxes that may be due.