You need to add the "side deck" metacategory to your card. You can also control how many challenge points the player will lose by choosing your side deck card by setting the "SideDeckValue" extended property to the appropriate number of points (by default, the player will not lose any points by choosing a side deck card, but if your card is better than a squirrel, the player should probably get some points taken off of their challenge rating if they choose it).

public static readonly CardMetaCategory SIDE_DECK_CATEGORY = GuidManager.GetEnumValue<CardMetaCategory>("zorro.inscryption.infiniscryption.sidedecks", "SideDeck");

// sometime later...
CardInfo myCard = ...;
myCard.AddMetaCategories(SIDE_DECK_CATEGORY);
myCard.SetExtendedPropert("SideDeckValue", 5);

If you are using JSON Loader:

{
    "name": "MyCard",
    "metaCategories": ["zorro.inscryption.infiniscryption.sidedecks.SideDeck"],
    "extensionProperties": {
        "SideDeckValue": "5"
    }
}