UGameplayTagsManager::Get().RequestGameplayTag(TEXT("Some.Tag.FName"))
The function has a second optional parameter telling it whether or not it should throw a tantrum if the tag in question isn’t found.
Edit:
If you actually want to CREATE a new tag, rather than look up an existing one, you can use:
UGameplayTagsManager::Get().AddNativeGameplayTag("TagName")
…but this can only be used during engine initialization.