i need to tag some one with fluentcrm after review is submitted

Not sure I understand. Please explain what you are trying to accomplish and for what purpose. There may be a better way to do it.

i need to know when someone sends a review via the WPreviewSlider form.and send it to a CRM or a connector tool like n8n.

Sorry, that currently isn't built in to the plugin. I can add it to my list of feature requests. In the meantime, you can get it to send an email when you get a review, then you could use some sort of automation tied to your email account.

Do you have any hook or filter where we can add a custom PHP snippet about the review submit?

There are no hooks or filters built into the plugin for that. All the reviews are stored in a custom table in your wp database, so you could just code something that looks at that table on a cron job and send any updates along.