Sitecore (headless) Forms custom field factory with hidden field using query string value provider. Sets the value of the hidden field as a given value from the query string.

custom-field-factory.tsx

import { useRouter } from 'next/router';
import { 
    FieldViewModel,
    InputViewModel
} from '@sitecore-jss/sitecore-jss-forms';
import {
    createDefaultFieldFactory,
    ValueFieldProps
} from '@sitecore-jss/sitecore-jss-react-forms';

export interface ValueProviderViewModel extends FieldViewModel {
    valueProviderSettings: {
        valueProviderItemId: string;
        parameters: string;
    };
  }
  
export interface ValueProviderInputViewModel extends InputViewModel, ValueProviderViewModel {}
  
const CustomFieldFactory = createDefaultFieldFactory();

// GUID for Hidden field - /sitecore/system/Settings/Forms/Field Types/Basic/Hidden
CustomFieldFactory.setComponent('{B67C70C1-6776-4BCB-8529-AA28E02F6412}', ({ field }: ValueFieldProps<ValueProviderInputViewModel>) => {

    const router = useRouter();

    // GUID is for Query String Value Provider 
    // https://github.com/bartverdonck/Sitecore-Forms-Extensions/blob/master/src/Feature/FormsExtensions/items/ValueProviders/Value%20Providers/URL%20Parameter%20-%20Query%20String.yml
    if(field.model.valueProviderSettings.valueProviderItemId == '{ACF6ABAD-6B2C-4294-808B-02FED11552A2}')
        field.model.value = router.query[field.model.valueProviderSettings.parameters] as string;

    return (
        <input type="hidden" id={field.valueField.id} name={field.valueField.name} value={field.model.value as string} />
    );
});