If you cannot or do not want to sign the law proposal online, you can send a signed statement of support ("Kannatusilmoitus") to the representative of this citizens' initiative.
Proceed as follows:
Your personal information is only given to the Population Register Centre for final check-up and count. The Finnish citizens' initative act states the following:
Personal information collected in the statements of support may only be used in accordance with the provisions of this act. The originator of the initiative and their agent can release the information only to the Population Register Centre. The information held by the authorities becomes public, when the minimum number of statements stated in the constitution has been fulfilled.
Kansalaisaloitelaki (12/2012) (Unofficial translation)