I staked on forum following addresses:
18GTv1CFmMTVCyzcynB5Y9RPxy89vGHdFA - From 1Broker
1P2zMTqBGdzvbyojzzBHEKm1Np8ACa8Zi3 - From Yobit
Sign a message from this wallet is not allowed, so how i coul confirm in future that i own this addresses yet?
It's useless to stake those addresses even if you were able to sign a message with them. The whole point of staking an address on BCT is to prove that you, as a BCT member who owns a particular account, are also in possession and in sole control of a unique private key. Since the people over at Yobit and 1Broker also have unfettered access to the aforementioned addresses (and thus the corresponding private keys), then any signed message from them would prove nothing.
Additionally, altcoin exchanges reserve the right to replace the deposit addresses of their users when it is deemed necessary. Some exchanges even allow users to voluntarily replace their deposit addresses. You know how some exchanges tell you beforehand not to mine to your deposit address or to always check your deposit address before sending coins to them -- that's in case the deposit address was replaced due to some unforeseen circumstance.
Eseentially, any signed message from an altcoin exchange's deposit address would not prove ownership of anything.
P.S. Ignore the earlier advices which don't know what they're talking about.