This is Server only functionality, because this options are send to the GameMode, and there is not GameMode client-side.
You have two options:
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When the client connects to the server, send that variable (OptionsString) to the client using the PlayerController for example
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Instead of using OptionsString, save that data in your ULocalPlayer (create a UMyLocalPlayer) that is not destroyed when travelling.
Additionally, if you need to send that data to the server, you can override GetGameLoginOptions() to send that info to the host’s game mode at very first moment previous to the creationg of your Controller/Character server side.