First add Mermaids to your Project:

Maven pom.xml:

<dependencies>
    <dependency>
        <groupId>plugin.siren</groupId>
        <artifactId>Mermaids</artifactId>
        <version>1.4.0</version>
        <scope>provided</scope>
        <optional>true</optional>
    </dependency>
</dependencies>

Gradle build.gradle: (untested)

dependencies {
    compileOnly 'plugin.siren:Mermaids:1.4.0'
}

Next, update the Hytale manifest:

Optional Dependency manifest.json:

"OptionalDependencies": {
  "Siren:Mermaids": "*"
}

Required Dependency manifest.json:

"Dependencies": {
  "Siren:Mermaids": "*"
}

Official project that used Mermaids as a Dependency:

Check out the source code for Orbis Origins X Mermaids mod!