How to specify a default goal for a Maven plugin?

I've defined a Maven plugin with multiple goals. Currently users run my plugin as follows:

<plugin>
    <groupId>myGroupId</groupId>
    <artifactId>myArtifactId</artifactId>
    <version>someVersion</version>
    <executions>
        <execution>
            <goals>
                <goal>myGoal</goal>
            </goals>
        </execution>
    </executions>
</plugin>

but I've seen other plugins, like maven-compiler-plugin and Flyway, that don't require specifying an execution: https://flywaydb.org/getstarted/java

<plugin>
    <groupId>org.flywaydb</groupId>
    <artifactId>flyway-maven-plugin</artifactId>
    <version>5.2.4</version>
    <configuration>
        <url>jdbc:h2:file:./target/foobar</url>
        <user>sa</user>
        <locations>
            <location>classpath:db/migration</location>
        </locations>
    </configuration>
</plugin>

How do I specify the goal that should run by default when users exclude the <executions> block?

AFAIK, there are not default goals for Maven plugins.

You can configure a plugin without adding a goal. But this does not execute the plugin.

The plugin must be either executed explicitly on command line (like flyway:migrate) or is executed automatically through the lifecycle (like compile:compile or jar:jar).