Lead should fill up data set

At the moment I am working step by step on moving all email lists (first name/email) from an external App into the Zoho CRM as "LEADS".

How exactly do I implement this so that I send the respective contact for whom I have either
- only email or
- email and first name
an email/link so that they update their existing LEAD profile with me (e.g. by adding their address, city, industry)

I had already created a "SELF SERVICE" form via FORMS, but then they would have to fill in all the missing fields/data again and then the user would possibly be in the CRM twice as a LEAD.

Does anyone have a tip/idea/implementation?

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