Controller:                                                                                                                                                                                 
  @http.route('/registro', type='http', auth='public', website=True)
  def registro(self, servicio=None, **kwargs):                                                                                                                                                
      return request.render('your_module.registro_page', {'servicio': servicio})                                                                                                              
  
Template:                                                                                                                                                                                   
  <span t-if="servicio" t-att-data-for="form_id"
        t-att-data-values="{'Asunto': servicio}" class="d-none"/>

Hi everyone, thanks for the insights!

Following the suggestion regarding the dynamic rendering of the website form controller, I've moved away from simple ID selectors to the more stable data-attribute-name="name" approach. I also understand that the OWL framework might be overwriting the value if the timing isn't perfect.

I’ve implemented a MutationObserver combined with a Native Value Setter to ensure the framework registers the change as a user-initiated event. However, I’m still seeing a 'flicker' where the value is injected but then cleared by Odoo's internal state management.

Below is the current iteration of the script I'm using. I've added a retry burst (polling) to fight against the framework's cleanup. Does anyone have experience with a specific OWL event or a website_form method that should be called to 'commit' this value to the component state properly?

<script>
(function () {
    const DEBUG_TAG = "Brissa AutoFill: ";
    
    const getServiceParam = () => {
        const params = new URLSearchParams(window.location.search);
        return params.get('servicio');
    };

    const fillField = (input, value) => {
        if (!input || !value) return;
        try {
            const decodedVal = decodeURIComponent(value);
            // Native setter bypasses OWL's virtual DOM sync issues
            const valueSetter = Object.getOwnPropertyDescriptor(window.HTMLInputElement.prototype, 'value').set;
            valueSetter.call(input, decodedVal);

            // Triggering multiple events to ensure Odoo's controller detects the change
            ['input', 'change', 'blur', 'focusout'].forEach(evt => {
                input.dispatchEvent(new Event(evt, { bubbles: true }));
            });
            console.log(DEBUG_TAG + "Syncing value: " + decodedVal);
        } catch (e) {
            console.error(DEBUG_TAG + "Injection Error", e);
        }
    };

    const findInput = () => {
        // Community recommended stable selector
        return document.querySelector('.s_website_form_field[data-attribute-name="name"] .s_website_form_input') 
               || document.querySelector('input[name="name"]');
    };

    const observer = new MutationObserver((mutations, obs) => {
        const input = findInput();
        const servicio = getServiceParam();

        if (input && servicio) {
            // Burst strategy: fill multiple times during the first 3 seconds
            // to prevent the 'Form Controller' from clearing the input.
            [0, 500, 1000, 2500].forEach(ms => setTimeout(() => fillField(input, servicio), ms));
            
            obs.disconnect(); 
        }
    });

    observer.observe(document.body, { childList: true, subtree: true });
})();
</script>

You’re facing this issue because Odoo website forms are rendered dynamically by the website form controller. Sometimes the form fields are not fully available when your script runs, so the selector either fails or the value is not registered when the form is submitted.

A simpler and more reliable approach is to run the script after the page is fully loaded and select the field using its name attribute.

Example:

window.addEventListener('load', function () {
const params = new URLSearchParams(window.location.search);
const service = params.get('servicio');

if (service) {
const field = document.querySelector("input[name='name']");

if (field) {
field.value = service;
field.dispatchEvent(new Event('input', { bubbles: true }));
field.dispatchEvent(new Event('change', { bubbles: true }));
}
}
});

This works better because the script waits until the form is fully rendered, and using input[name='name'] is more stable than relying on dynamic IDs like Field. Triggering the input and change events also ensures that Odoo properly detects the value before the form is submitted.

Another common approach in Odoo websites is to use a hidden field in the form and populate it from the URL parameter, which is often cleaner for tracking things like lead source or campaign parameters.