> ## Documentation Index
> Fetch the complete documentation index at: https://fillout.com/help/llms.txt
> Use this file to discover all available pages before exploring further.

# Advanced Fillout Guides

> Advanced guides on Fillout's intricate features and capabilities.

## Pre-filling

<CardGroup cols={2}>
  <Card color="#FFC738" icon="fill" iconType="solid" href="/prefill-fields" title="Pre-fill fields">
    Set default values using past inputs, URL parameters, and more.
  </Card>

  <Card color="#FFC738" icon="calendar-days" iconType="regular" href="/pre-fill-dates" title="Pre-fill dates">
    Set the default value to today, tomorrow, or another date.
  </Card>

  <Card color="#FFC738" icon="list-dropdown" iconType="regular" href="/dynamic-form-dropdowns" title="Populate dropdown options with a webhook">
    Dynamically populate from a REST API.
  </Card>

  <Card color="#FFC738" icon="brackets-curly" iconType="regular" href="/url-parameters" title="URL parameters/hidden fields">
    Pass information to a form to support pre-filling and personalization.
  </Card>

  <Card color="#FFC738" icon="file-brackets-curly" iconType="regular" href="/url-parameters-between-forms" title="Form parameters">
    Send data from one form to another with URL parameters.
  </Card>
</CardGroup>

## API

<CardGroup cols={2}>
  <Card color="#FFC738" icon="code" iconType="regular" href="/fillout-rest-api" title="Fillout REST API">
    Access your forms and submissions outside the Fillout site.
  </Card>

  <Card color="#FFC738" icon="grid-2-plus" iconType="regular" href="/oauth-applications" title="Create Fillout 3rd Party Apps">
    Create applications to connect Fillout to 3rd party applications.
  </Card>
</CardGroup>

## Other

<CardGroup cols={2}>
  <Card color="#FFC738" icon="rectangle-code" iconType="regular" href="/custom-code" title="Custom code">
    Add custom code for analytics tracking like Google Tag Manager.
  </Card>

  <Card color="#FFC738" icon="quote-left" iconType="regular" href="/custom-values" title="Add custom values to answer options">
    Set distinct values for options, separate from the labels visible to respondents.
  </Card>

  <Card color="#FFC738" icon="shuffle" iconType="regular" href="/randomize-options" title="Randomize options">
    Shuffle the order in which answers or choices are displayed.
  </Card>

  <Card color="#FFC738" icon="arrow-right-to-line" iconType="regular" href="/right-to-left-forms" title="Right to left aligned forms">
    Make surveys or forms aligned in the right-to-left direction.
  </Card>

  <Card color="#FFC738" icon="link" iconType="regular" href="/add-links-to-form" title="Add links and external URLs">
    Add clickable URLs in your form to link to external websites or services.
  </Card>

  <Card color="#FFC738" icon="screen-users" iconType="regular" href="/collaborative-forms" title="Collaborative forms">
    Send a single, shared form to multiple respondents at once.
  </Card>

  <Card color="#FFC738" icon="inbox-out" iconType="regular" href="/smtp" title="Use SMTP">
    Send custom emails using your SMTP server to ensure reliable delivery.
  </Card>
</CardGroup>