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Your LimeSurvey version: [ 3.25.16+210302]
Own Server or LimeSurvey hosting: Own Server
Survey theme/template:
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I am conducting a study with many participants (+20 000). I am trying to find several ways to increase the response rate. The survey is closed to certain participants from who I have a database. With information from the database, I apply some filters to the questions. In brief, it has to be closed. Consequently, I can not post a generic URL everyone can access because each participant has a link with their token. 
I would like to share on the community website where we are doing the study a unique link everyone can access and the platform to identify them so they answer their specific questions. I thought of creating a token based on the information I have and each participant has so they can access the survey's welcome site and enter it, but this has some privacy problems.
I came up with a possible solution but I do not know if it is possible with the current system. I would like to have a survey welcome website where the participant can type their email and if they are in the participant's database, the system would send the invite or reminder to their email addresses. The advantage is that sometimes people ignore the normal invite or reminder whereas in this scenario there are willing to open it.

I hope this is clear enough. Thanks

Except with plugins i don't see how you can do this …