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https://steemconnect.com/sign/account-witness-vote?witness=swisswitness&approve=1

this is the link to vote for my witness for example : @swisswitness
if you change the name in the link to curie then you get

https://steemconnect.com/sign/account-witness-vote?witness=curie&approve=1

and so on, you can check witnesses here, this is the fullest list i know of
https://steemian.info/witnesses