First of all you should go to the main pages of different recent dramas and find the names of moderators of your language. The team members can all be seen if you click on the team name.
Then write a message to each of them (in Spanish, of course), telling them that you’re willing to translate if they have a project that needs help.
Try to write a well-written message, because this shows your fluency in the language and your care in expressing yourself, with correct punctuation etc.
To contact them: https://support.viki.com/hc/en-us/articles/200138974-How-do-I-message-a-Channel-Manager-or-Moderator-
You may want to check this thread about beginners and the Project Finder
this one with helpful advice for beginners
this post too:
this one about the Project Finder and its problems