Where should I set aspectVersion to use aspejct.gradle plugin?

aspectj.gradle plugin mandate to set aspectJVersion before applying it. Hence, I write my code as below. However, only buildscript {} and other plugins {} are allowed to show before plugins {}. So, where should I set apsectJversion in my build? PS. I am using gradle 5.0

project.ext {
aspectjVersion ‘1.8.9’

plugins {
id ‘aspectj.gradle’ version ‘0.1.6’