I want to pay taxes so that I can get offical loans like credit card and car loan
But my income are under the radar and dont have proper documentation with both parties
just report 2-liner trade income as required and pay your taxes due, I don't think anyone will get penalized for paying taxes for income earned without substantiate papers. The law is more targeted at tax evaders and people looking to under-report income or over claiming expenses. You are doing the total opposite here which is totally "legitimate".
You get what I mean? I can just state I earned 2 million in FY2018 (which in actual fact I didn't) but I still pay the tax bracket for 2 mil, the IRAS will be very happy and I don't think there any penalty for doing that. The trouble only starts when you earn really $2 million but you reported as $200K or claiming expenses at $1.8million out of the $2 mil earned.
Hope the logic sounds ok.

