Claiming it
Opening the link uses nothing: the page says how long the Plus runs. Sign in with ClackID and press Start to claim it. Nothing is charged and no card is needed.
Some links also give CW+ on CageWatchr for the same time. If you have not connected CageWatchr on your ClackID account yet, that switches on when you do.
If it cannot be used
Each link works once. If it says it cannot be used, it has been used, has expired or was cancelled.
If you already pay for Plus, or have it with CW+, the link is not needed and is left unused, so it can go to somebody else.
When it ends
Everything you made is kept, as with any Plus ending.