Edit search form with a module
Okay, I think there must have been a typo somewhere or something, but anyway, this setup works:
info yaml:
name: Search Icon
description: This module replaces search value by an icon.

type: module
core: 8.x
core_version_requirement: ^8 || ^9

module:
&lt;?php    

/*
 * Implementation of hook_form_alter()
 */    

function searchIcon_form_alter(&amp;$form, &amp;$form_state, $form_id) {
    if ($form_id == 'search_block_form') {
        $form['actions']['submit']['#value']= 'TESTSEARCH';
    }    
}

Please note that the module does not work as intended yet, the value should be replaced by an html decimal code for the desired icon.