where can i find line 35a? I want to apply some of my refund to next year instead of having the entire amount refunded to me.

Line 35a of your Form 1040 shows your federal refund

 

PREVIEW 1040

 

https://ttlc.intuit.com/questions/1901539-how-do-i-preview-my-turbotax-online-return-before-filing

Click on Tax Tools on the left side of the screen. Click on Tools. Click on View Tax Summary. Click on Preview my 1040 on the left side of the screen.