$fieldset->addField(
'first_name',
'text',
[
'name' => 'FirstName',
'label' => __('First Name'),
'id' => 'FirstName',
'title' => __('First Name'),
'required' => true,
'class' => 'input-text',
'data-validate' => '{"required":true, "validate-FirstName":true}',
'value' => $firstname // Set default value
]
);