Edit the files

1.) Go to include/tour-util.php file in tourmaster plugin folder
2.) Around line 1195, you'll see list of enquiry form fields.

$enquiry_fields = array( 
'full-name' => array( 
    'title' => esc_html__('Full Name', 'tourmaster'), 
    'type' => 'text', 
    'required' => true ), 
'email-address' => array( 
    'title' => esc_html__('Email Address', 'tourmaster'), 
    'type' => 'text', 
    'required' => true ), 
'your-enquiry' => array( 
    'title' => esc_html__('Your Enquiry', 'tourmaster'), 
    'type' => 'textarea', 
    'required' => true ),
);

3.) Try adding new field inside the array() as you want. 
4.) The structure will be ( select the "type" as "text" or "textarea" and select the "required" to "true" or "false" )

'FIELD_NAME' => array( 
    'title' => esc_html__('FIELD_TITLE', 'tourmaster'), 
    'type' => 'textarea/text',  
    'required' => true/false ),

Modifying the mail

1.) Go to "Tourmaster plugin option > General > Enquiry E-mail Content" section
2.) use {FIELD_NAME} to replace the word you want with the data user're filling.

Dear {full-name},
You have sumiited an enquiry from {tour-name}
Message: {your-enquiry}
Our team will contact you back via the email you provided, {email-address} 
Thank you!