Pass variable from custom module to twig template

Question

How can I pass a variable from my custom module to a twig field template?

my field template is the "field--node--field-webform--parliament-visit.html.twig"

and my module is named as "custommodule"

I tried this function according to documentation but apparently I didnt understand smth correctly:

  function custommodule_preprocess_field_node_field_webform_parliament_visit(array &$variables) {
    $VARIABLE_NAME = 'my_variable';
    $variables['varname'] = $VARIABLE_NAME ;
  }