Sometimes we can have a requirement to add or modify field values in onsubmit event of lightning:recordeditform. In below example, I’m creating “Lead” object record using lightning:recordeditform and in onsubmit event, I’m adding “Description” field value of “Lead” object.

Lightning Component:

<!--Sample.cmp--> 
<aura:component implements="flexipage:availableForAllPageTypes,force:appHostable">
    
    <!--Component--> 
    <div class="slds-m-around--xx-large">
        <lightning:card title="Lead" iconName="standard:lead" class="slds-p-around_medium">
            <lightning:recordEditForm aura:id="leadCreateForm" objectApiName="Lead" onsubmit="{!c.handleOnSubmit}">
                <lightning:messages />
                
                <div class="slds-grid">
                    <div class="slds-col slds-size_1-of-2 slds-p-around_medium">
                        <lightning:inputField fieldName="FirstName"></lightning:inputField>
                    </div>
                    <div class="slds-col slds-size_1-of-2 slds-p-around_medium">
                        <lightning:inputField fieldName="LastName"></lightning:inputField>
                    </div>
                </div>
                
                <div class="slds-grid">
                    <div class="slds-col slds-size_1-of-2 slds-p-around_medium">
                        <lightning:inputField fieldName="Email"></lightning:inputField>
                    </div>
                    <div class="slds-col slds-size_1-of-2 slds-p-around_medium">
                        <lightning:inputField fieldName="Phone"></lightning:inputField>
                    </div>
                </div>
                
                <div class="slds-grid">
                    <div class="slds-col slds-size_1-of-2 slds-p-around_medium">
                        <lightning:inputField fieldName="Company"></lightning:inputField>
                    </div>
                </div>
                
                <lightning:button type="submit" label="Save" variant="brand"/>
            </lightning:recordEditForm>
        </lightning:card>
    </div>
</aura:component>

Lightning JS Controller:

({
    handleOnSubmit : function(component, event, helper) {
        event.preventDefault(); //Prevent default submit
        var eventFields = event.getParam("fields"); //get the fields
        eventFields["Description"] = 'Lead was created from Lightning RecordEditForm'; //Add Description field Value
        component.find('leadCreateForm').submit(eventFields); //Submit Form
    }
})