Set the meeting url using a flow

Go to Setup > Flows and create a new Flow

Select Record-Triggered Flow

Select Reservation as the object, leave the defaults and at the bottom select “Include a Run Asynchronously path to access an external system after the original transaction for the triggering record is successfully committed”

Create a Reservation Collection variable

Under Run Asynschronously, click the +

Create an Assignment as follows

Next, add an Action

Select Create Zoom meetings for list of reservations

Set the input value reservations to your Reservation Collection

Set the input value for "zoomUserId" to the email address of the Zoom user you want to designate as the creator of the meeting, this can be a fixed one or use a field somewhere on the resource, staff etc you’re booking to dynamically do this.

Save and activate the flow

The flow will now fill the “Zoom_Join_Url__c” field on the Reservation object asynchronously depending on your configuration.