Use the class name. For example:

latestEvent.map(AssetEvent::getTimestamp).ifPresent(latestList::add);

Assuming AssetEvent is the name of the class having the getTimestamp method.

A method reference (List::forEach) can't take an argument so you can't compose them. On the other hand you can pass a method reference as an argument to a "normal" method call (l.forEach(BufferedImage::flush)).

I don't think you can do much better than your second to last option.

What about use enum to declare all fields from Request and use it as common part of the code. I did not check it, this is only to show my approach:

public enum RequestField {
    PLANT_ID(Request::getPlantId, (val, campaign) -> campaign.plant.id::contains),
    TITLE(Request::getTitle, (val, campaign) -> campaign.title::containsIgnoreCase),
    CAMPAIGN_NUMBER(Request::getCampaignNumber, (val, campaign) -> campaign.campaignNumber::like),
    // ... more fields here ...
    ;

    private final Function<Request, Optional<Object>> get;
    private final BiFunction<Object, Campaign, BooleanExpression> map;

    RequestField(Function<Request, Object> get, BiFunction<Object, Campaign, BooleanExpression> map) {
        this.get = get.andThen(Optional::ofNullable);
        this.map = map;
    }

    public static List<BooleanExpression> getBooleanExpressions(Request request, Campaign campaign) {
        if (request == null)
            return Collections.emptyList();

        List<BooleanExpression> res = new LinkedList<>();

        for (RequestField field : values())
            field.get.apply(request)
                     .map(r -> field.map.apply(r, campaign))
                     .ifPresent(res::add);

        return res.isEmpty() ? Collections.emptyList() : Collections.unmodifiableList(res);
    }
}

And your client code will be looking like:

List<BooleanExpression> booleanExpressions = RequestField.getBooleanExpressions(request, campaign);

P.S. Your last code could be look like:

if (request.getLockVehicle() != null)
    expressionMapping.add(request.getLockVehicle() ? campaign.lockVehicle.isNotNull() : campaign.lockVehicle.isNull());