How to fix this squeak sound ?

[sunny]

It's moisture trapped inside the fork around the seal I believe. I had it on my Yamaha 1100 from riding in the rain. It took a few weeks to a month of everyday riding for it to disappear if I remember correctly. It wasn't constant just every time I slowed and used the front brake with the transfer of motion forward. It may in fact be the mudguard just going through the process of elimination. The WD 40 around the seals (like TURBO200R4 stated) may help and be generous with it. It's worth a try anyway. If it doesn't work it was TURBO's idea.

Before I rode around 50km I lubed a lot with wd40 + car engine oil on the seal, forks but the squeak is still there.

I have not opened the mudguard which I will do probably tomorrow and confirm.

if its not the mudguard then I guess you are right, it will take whatever it will take to disappear by itself or I will live with it