Check out the module "currency rate" by intelliversal integrated solutions. 

Appreciated

To get a currency rate:

• ​Invoicing/Configuration/Currencies   [or]
• Accounting/Configuration/Currencies

Just Activate which country currency you want!

And to display the currency in a form use,

in Python file:

currency_id = fields.Many2one('res.currency', string='Currency', default=lambda self: self.env.company.currency_id)

and in xml file:

add fields currency_id and fields amount with a widget="monetary" in the respective xml

Don't forgot to add "currency_id" , because sometimes if it  is not present , then  does not display the symbol of the currency