If you are planning on sending emails to your existing/past guests using a mailing service, then you can create a special rate - and use the options for an AGENT code, https://wiki.beds24.com/index.php/Category:Agents
There is a few things that you need to setup, it is all in this wiki page
Go to Settings > Properties > Booking Rules > Agent Booking Page > Enter your Agent Code for example : Existing
Go to Settings > Properties >Payment Collection, set Deposit 2 = 50
Create a new Rate with Booking Type = Deposit 2, Go to the Channel Tab and select only - www.beds24.com
Then you can send a link in the email to your guests with the URL to your booking page and the Agent code added
Your normal Booking page URL = https://www.beds24.com/booking2.php?propid=123456
then you add the Agent code details &agent=Existing
Then you have = https://www.beds24.com/booking2.php?pro ... t=Existing
By adding &referer=ExistingCustomer (or any text you prefer) to the end of the URL you will also be able to track any bookings that have been generated by existing guests.
So you can use this URL :-
https://www.beds24.com/booking2.php?pro ... ngCustomer
If you use any software (like bitly.com) that can shorten URL links, you can mask the URL if you prefer, have a look on the internet and search for 'Shorten URL Links'