IFRS 15, Revenue from Contracts with Customers, provides guidance on the accounting for revenue from contracts with customers.
In your case, the stamp tax is a cost incurred in obtaining the contract and would be recognized as an expense immediately when the contract is signed, regardless of when the revenue is recognized. The IFRS 15, Paragraph B58 states that "costs of obtaining a contract are recognized as an expense when incurred if the amortization period of the assets that the entity otherwise would have recognized is expected to be one year or less."
However, the company can expense the stamp tax in line with the revenue stream if the company can demonstrate that the stamp tax is directly related to the revenue generated by the contract, it would be more appropriate to recognize the expense on the same basis as the related revenue.
The company should also disclose the nature, amount, and expected timing of revenue and related costs in the notes to the financial statements.
It's worth noting that the fact that the stamp tax is irrecoverable even if the contract is cancelled, it does not affect the accounting treatment of the stamp tax as an expense.