How can I get a proof of residency from the Thai Tax Department for Portuguese authorities?

Are you being asked by Portuguese Tax Authorities to prove you're a resident of Thailand for tax purposes? Or is it some other Portuguese Authority that wants to know your status? If it's for Portuguese Tax Authorities, then based on comments by

@Pete ******

all you could do until now, is tell those authorities that you are residing longer term in Thailand, with proof (DTV & maybe rental contract, plus maybe a residence certificate from your local Thai Immigration Office) and advise the Portuguese that you can't provide any certificate from the Thailand Revenue Department until 2026,after you've lodged a Thai tax return. I'm guessing that you'd prefer to be taxed in Thailand rather than Portugal.

Are you on a DTV ?

Here are the requirements to get one yourself although I would first try using your TIN and passport as proof of tax residency.

the "certificate of residency" is from Thai Immigration, not from the Tax Revenue Department