MissingPropertyException: Gradle Task Fails Due to Unknown Property

I am encountering the following error:

    Caused by: groovy.lang.MissingPropertyException: Could not get unknown property 'integrationTest' for SourceSet container of type org.gradle.api.internal.tasks.DefaultSourceSetContainer.

This is my build.gradle file. I have a integrationTest task defined already.

plugins {
	id 'java'
	id 'org.springframework.boot' version '2.7.8'
	id 'io.spring.dependency-management' version '1.0.15.RELEASE'
}

group = 'com.app.api'
version = '0.0.1-SNAPSHOT'
sourceCompatibility = '17'

repositories {
	mavenCentral()
}

dependencies {
	implementation 'org.springframework.boot:spring-boot-starter'
	implementation 'org.springframework.boot:spring-boot-starter-web'
	implementation 'com.fasterxml.jackson.datatype:jackson-datatype-jsr310:2.11.0'
	testImplementation 'org.springframework.boot:spring-boot-starter-test'
	testImplementation 'junit:junit:4.13.1'
}

tasks.register('integrationTest', Test) {
	testClassesDirs = sourceSets.integrationTest.output.classesDirs
	classpath = sourceSets.integrationTest.runtimeClasspath
}

tasks.withType(Jar).configureEach {
	duplicatesStrategy = DuplicatesStrategy.EXCLUDE

	manifest {
		attributes["Main-Class"] = 'com.app.api.ApiApplication'
	}

	configurations["compileClasspath"].each { file ->
		from(zipTree(file.absoluteFile))
	}
}

How can one make integrationTest property be known to gradle?

Using the ext block (which allows you to define custom properties and methods that can be accessed from anywhere in your build file), I can add the custom property integrationTest

ext {
    integrationTest = project.hasProperty('integrationTest')
}

And I added a check in the integrationTest task:

tasks.register('integrationTest', Test) {
    if (integrationTest) {
        testClassesDirs = sourceSets.integrationTest.output.classesDirs
        classpath = sourceSets.integrationTest.runtimeClasspath
    }
}

Now when I add the system property via the terminal:
./gradlew integrationTest DintegrationTest=true

The property's value will be set to true.