I have an I phone 8 plus red and the back screen cracked I have a warranty by how do I get it fixed with warranty

The standard warranty does not cover cosmetic damage like cracks, you would need to get it repaired at cost. If you have Apple Care + as an additional warranty Apple will provide two instances of accidental damage coverage where the cost to replace the screen is greatly reduced.

https://support.apple.com/iphone/repair

If you are unsure of your warranty or warranty status use this link:

Check Your Service and Support Coverage - Apple Support

If it's the back glass then it's £356.44.

https://support.apple.com/en-gb/iphone/repair

"...and if I do can I use that for a fixture"

No.

As William said, if you went to buy AppleCare+ now, they will want to inspect your phone. Since your back glass was broken before AppleCare+, they will either deny you, or exclude the back glass from being covered.

Note also that adding AppleCare + after purchase requires an examination of the phone, so you won't just be able to get it fixed for £149 by adding AppleCare + now.

Hey kunal here

I have buy a new iPhone 8plus i want to know that my phone is in warranty or not