Use a Screen Flow to Pass Input Data to a Dynamic WhatsApp Flow

For example, the FormData parameter stores this input data.

{“FirstName”:“Albert”,“LastName”:“Costa”,“isStudent”:“true”}

Dynamic references can pull data from data model fields or input fields defined in a screen.

This Apex class transforms structured input data—a user's name and a list of Salesforce records—into a single JSON string. This JSON string is then used to pass input data to the Apex-based form messaging component.

public class FlowJSONBuilder {

    // 1. Input parameters
    public class FlowInput {
        @InvocableVariable(label='User Name' required=true)
        public String meuName;

        // CHANGE: Use List<SObject> instead of List<DynamicFlowItems__c> to avoid compile error
        @InvocableVariable(label='Flow Items' required=true)
        public List<SObject> items; 
    }

    // 2. Output parameters
    public class FlowOutput {
        @InvocableVariable(label='Generated JSON')
        public String jsonString;
    }

    // 3. Internal Wrapper classes
    public class JSONStructure {
        public String greeting;
        public List<ItemWrapper> items;
    }

    public class ItemWrapper {
        public String id;
        public String title;
    }

    // 4. Main Method
    @InvocableMethod(label='Generate Custom JSON' description='Builds the JSON string from Flow items')
    public static List<FlowOutput> generateJson(List<FlowInput> requests) {
        List<FlowOutput> outputs = new List<FlowOutput>();

        for (FlowInput request : requests) {
            FlowOutput output = new FlowOutput();
            
            JSONStructure jsonRoot = new JSONStructure();
            jsonRoot.greeting = 'Hello ' + request.meuName + '!';
            jsonRoot.items = new List<ItemWrapper>();

            if (request.items != null) {
                // Loop through generic SObjects
                for (SObject genericRec : request.items) {
                    // CAST the generic SObject to your specific Custom Object
                    DynamicFlowItem__c rec = (DynamicFlowItem__c)genericRec;
                    
                    ItemWrapper item = new ItemWrapper();
                    item.id = rec.itemId__c;     
                    item.title = rec.itemTitle__c; 
                    jsonRoot.items.add(item);
                }
            }

            output.jsonString = JSON.serialize(jsonRoot);
            outputs.add(output);
        }

        return outputs;
    }
}

At run time, the screen action invokes the Apex-based form messaging component and passes this input data to it.