Question for BOTSWANA UNIFIED REVENUE SERVICE (HEAD OFFICE)

How we can apply a tax exemption?
Percival Software Ltd is participation in tender: Botswana Stock Exchange. A service contract for supply, installation, configuration, migration and support of a Depository, Clearing and Settlement System. According to the requirements we are obligated to apply tax exemption from your office.
Percival Software Ltd is registered in United Kingdom, registration number ********.
As per Convention for the Avoidance of Double Taxation signed between Botswana and UK any tax shouldn't be withheld from payments to Percival Software Ltd.