LWC Cmp button:

<lightning-button variant="brand" type="submit" label="Save" class="slds-m-left_x-small"
                        onclick={handleSubmit}>
                    </lightning-button>


controller code:

handleSubmit(event) {
    event.preventDefault();
    const fields = event.detail.fields;
    fields.Person_Involved__c = this.personInvolvedId;
    fields.Investigation__c = this.investigationId;
    this.template.querySelector('lightning-record-edit-form').submit(fields);
}