How to add dynamic fields to "onsubmit" method in lightning-record-form in LWC

Question

I'm new to Lightning Web Components development. I have 3 fields to populate values, out of which 2 are picklist fields which requires user input, to which i use lightning-record-form with "edit" mode and the other field should not be displayed to end user but should be populated automatically.

My sample code (adding only the necessary parts):

Deal.html

<template>
    <lightning-quick-action-panel header="Finance">
        <div class="slds-p-around_large">
            <lightning-record-form record-id={recordId}
                                   object-api-name={apiName}
                                   columns=1
                                   mode="edit"
                                   fields={fields}
                                   onsubmit={handleSubmit}>

            </lightning-record-form>
        </div>
    </lightning-quick-action-panel>
</template>

Deal.js

/* eslint-disable no-console */
import { LightningElement , api} from 'lwc';
import { ShowToastEvent } from 'lightning/platformShowToastEvent';
import { CloseActionScreenEvent } from 'lightning/actions';

import DEAL_OBJECT from '@salesforce/schema/Deal__c';
import DEAL_A from '@salesforce/schema/Deal__c.Picklist_A__c'
import DEAL_B from '@salesforce/schema/Deal__c.Picklist_B__c'
import DEAL_FINANCE from '@salesforce/schema/Deal__c.Finance__c'

export default class Deal extends LightningElement {

    @api recordId;
    apiName = DEAL_OBJECT.objectApiName;
    fields = [DEAL_A, DEAL_B];

    handleSubmit(event){
        // Stops the submit action
        event.stopPropagation();
        event.preventDefault(); 

        let updated_fields = event.detail.fields;
        console.log(JSON.stringify(updated_fields));

        // Do validation checks

        if(!updated_fields.Picklist_A__c){
            this.myToastEvent("Error !!","Please enter a value","error");
        }
        else if(!updated_fields.Picklist_B__c){
            this.myToastEvent("Error !!","Please enter a value","error");
        }
        else{
            // If validation passes, I need to add the 3rd field - Checkbox to true and Save
            /* I tried this code , but it did not work
            //event.detail.fields.push({"Finance__c":true});

            /* I tried this code , but it did not work
            //this.fields[DEAL_FINANCE] = true;
            
            /* I want to add this DEAL_FINANCE field without displaying it in lightning-record-form

            this.template.querySelector('lightning-record-form').submit(updated_fields);
            this.myToastEvent("Success !!","Deal Records updated successfully","success");
            this.dispatchEvent(new CloseActionScreenEvent());
        }
    }

    myToastEvent(mytitle,mymessage,myvariant){
        this.dispatchEvent(
            new ShowToastEvent({
                title: mytitle,
                message: mymessage,
                variant: myvariant
            })
        )
    }
}

META XML uses lightning__RecordAction as target and ScreenAction as targetconfig

Answer 1

From what I see, the 3rd field is not yet added to the array. So the first thing you can try to do is this.fields.push(DEAL_FINANCE); After this I would suggest setting the value property to true for that field.