Post values and problem with Value in form

I am trying to pass data from a legacy application in flash to a form in Drupal 8.

When I try to set the value of a field in the form , it does nothing with it. I am using this as a test so I can POST a value from flash and add it to a field in the form. This is part of the php

public function getFormId() {
    return 'details_form';
  }

$test = '123';

 public function buildForm(array $form, FormStateInterface $form_state) {

...
 $form['participant_school'] = array(
      '#type' => 'textfield',
      '#title' => t('Fantastic School:'),
      '#value' =>$test,
      
    );

}

$values = array(

'participant_school' => $test,

);

 $insert = db_insert('mytable')
		-> fields(array(
'school' => $values['participant_school'],
))
		->execute();

any guidance is very much appreciated

Drupal version: 


Source: https://www.drupal.org/taxonomy/term/4/feed