How do I use Hamcrest with JUnit 5 when JUnit 5 doesn't have an assertThat() function?

You have to make sure Hamcrest is included in the classpath and then use the assertThat() function provided by Hamcrest. From the current JUnit 5 User Guide - Writing Tests Assertions,

JUnit Jupiter’s org.junit.jupiter.Assertions class does not provide an assertThat() method like the one found in JUnit 4’s org.junit.Assert class which accepts a Hamcrest Matcher. Instead, developers are encouraged to use the built-in support for matchers provided by third-party assertion libraries.

The following example demonstrates how to use the assertThat() support from Hamcrest in a JUnit Jupiter test. As long as the Hamcrest library has been added to the classpath, you can statically import methods such as assertThat(), is(), and equalTo() and then use them in tests like in the assertWithHamcrestMatcher() method below.

import static org.hamcrest.CoreMatchers.equalTo;
import static org.hamcrest.CoreMatchers.is;
import static org.hamcrest.MatcherAssert.assertThat;

import org.junit.jupiter.api.Test;

class HamcrestAssertionDemo {

    @Test
    void assertWithHamcrestMatcher() {
        assertThat(2 + 1, is(equalTo(3)));
    }

}

Naturally, legacy tests based on the JUnit 4 programming model can continue using org.junit.Assert#assertThat."

See https://github.com/junit-team/junit5/issues/147:

you can use both, Hamcrest and AssertJ, in JUnit5. Both frameworks have a simple assertThat method, that you can import and use if wanted.

Currently, we do not plan to support these frameworks within our own Assertions to avoid the dependencies. Still, one can use them very well.