Hi,

We will use compute method for setting the value to the field, At first we need to fetch the sale order that we want to get the amount_total and date order from, to do that we will use the 'search' method and specify the domain for the record,

Try this code in the custom module:

test = fields.Char(compute='compute_test')

def compute_test(self):
"""Enter your domain in place of 'your_domain' """
    for rec in self:
        record = rec.env['sale.order'].search([your_domain])
        amount_total = record.amount_total
        date_order = record.date_order
        rec.test = str(amount_total)+str(date_order)

Regards