Netcode - Exception: Type Unity.Collections.FixedString32Bytes is not serializable

Hello again!

I was following some good online tutorials, trying to learn :) But once again I am stuck with a problem without solution: I can't define a NetworkVariable of type string, like the ones suggested on those tutorials:

private NetworkVariable FixedString32Bytes displayName = new NetworkVariable FixedString32Bytes() (yeah, I know the <> are missing...);

This is the result:

Exception: Type Unity.Collections.FixedString32Bytes is not serializable - it must implement either INetworkSerializable or ISerializeByMemcpy Unity.Netcode.NetworkVariableSerialization`1[T].WriteValue[TForMethod] (Unity.Netcode.FastBufferWriter writer, TForMethod& value) (at Library/PackageCache/com.unity.netcode.gameobjects@1.0.0-pre.8/Runtime/NetworkVariable/NetworkVariableSerialization.cs:92)

Good tutorial samples:
https://www.youtube.com/watch?v=OGVDIu7qOaE
https://www.youtube.com/watch?v=d1FpS5hYlVE

Anyone could help me with NetworkVariables and strings, please?

Thank you much!