With help from the awesome Ryan!
'profession' => [
'#type' => 'select',
'#title' => $this->t('Professional Category'),
'#required' => TRUE,
'#empty_option' => $this->t('- Select -'),
'#options' => $professions,
'#default_value' => isset($user) ? $user->get('field_profession')->target_id : '',
],
Profession is the custom user field.
:)