WHT has to be deducted and paid to govt by the lessee.
When you file personal tax return in Thailand, the WHT will be refunded to you, if your income does not attract tax for the FY. if your only income for the year in Thailand is less than 150,000, you will receive a full refund.
I once had a tenant who worked for an international airline co, and rent was paid by the airline. they would deduct WHT and i had to claim it at the end of year while filing tax returns.
In your case, if your tenant is not a corporate entity, they would not deduct WHT.
if your only income in Thailand is limited to 120,000baht, you don't have any tax exposure. just validate with a 'tax practitioner', if you are liable to file tax returns at the end of the year or otherwise.