If your want to attach the record ID of another Salesforce object to your Appointment Detail then use the Related ID URL parameter "rId" in the link your customer uses to book an appointment.
After the booking is made, the related object ID will be stored on the Appointment Detail object in Salesforce. And the Salesforce Event record will have the related object referenced in the “related to” field on the Event.
This parameter works for both standard and custom objects in Salesforce with one caveat:
Do not use this parameter for Lead, Contact, or Person Account objects. For those, use the similar Customer ID “cId” parameter. This parameter is discussed in this article and should be used for the objects Lead, Contact, or Person Account.
Here's how it works with an example:
Let's say your Appointiv online booking form link is http://yourcompany.appointiv.com
And let's say your favorite client, Acme Co, has an Opportunity saved in Salesforce with the record ID=0060K00000bMCj0QAG
We simply add the Salesforce Contact record ID onto the end of the booking form URL using this formula
NOTE 1 - The spelling and capitalization have to match so please notice that the operator is cId which is actually lower case "r" as in Roger - capital "I" as in India- lower case "d" as in Delta
NOTE 2 - You need to use the 18-digit version of the record ID. Salesforce uses 18-digit ID's now with Lightning but there are older 15-digit versions that sometimes get picked up in your internal formula fields. So if you're using a formula field on a record to add the ID then use CASESAFEID() in the formula. Here's a Salesforce article on it.
Mix and match your URL parameters to get even more customization and automation
It is also totally fine to mix and match parameters as needed.
Take the example above and say you also know this link will be sent to a specific Contact record. You can add both Customer ID and Related ID parameters together. In that scenario, the customer doesn’t need to fill their details on the booking form which makes their life easier, and the appointment and Event records get automatically related to the correct Opportunity (or other record) which makes your life easier.
Simply string the parameters together using the & command in the URL
Please note that only one Customer ID and one Related ID per link can be used.