Someone gave me .781 btc on that platform and i need to put 0.01 btc so i can withdraw all of it.
No, they didn't.
Every site/exchange/company/person/app/etc. which requires you to deposit coins before you are allowed to withdraw is a scam. Full stop. No exceptions. They are trying to scam you.
Some girl send .781 btc to me.
"Some girl" you don't even know just gifted you $43,000? Come on. Use your head. It is a fake platform that can display anything they want it to display. There is no bitcoin. They ask you to deposit, and if you are stupid enough to do it, then they will say the activation failed and you need to send more, or you only activated a low tier and you need to send more, or if you wait a week and then send more you'll get even more in return, or anything that will get you to deposit more. As soon as you stop depositing more, they will move on to their next victim.
Don't send them anything. You are being scammed.
In case you still don't believe me, the entirety of the text on the front page of their website is plagiarized from this exchange:
https://cex.io/Here is a list of sites which are completely identical to the one you are using, all trying to pull off the exact same scam:
https://www.skyescrm.com/https://www.haxbit.com/https://www.bitprotrade.com/https://trader-bull.com/https://trade.surf/https://mybitfx.com/