final List<String> suggestions = List.of("first_element", "another_element", "weee");
Commands.literal("customsuggestions")
    .then(Commands.argument("input", StringArgumentType.word())
        .suggests((ctx, builder) -> {
            suggestions.forEach(element -> {
                if (element.startsWith(builder.getRemainingLowerCase())) {
                    builder.suggest(element);
                }
            });

            return builder.buildFuture();
        })
        .executes(ctx -> {
            ctx.getSource().getSender().sendRichMessage("<gold>You selected <input>!",
                Placeholder.unparsed("input", StringArgumentType.getString(ctx, "input"))
            );
            return Command.SINGLE_SUCCESS;
        })
    ).build();