Code:

<?xml version="1.0" encoding="UTF-8"?>
<template>
<!-- Inherit the web.CalendarQuickCreate template -->
<t t-inherit="web.CalendarQuickCreate" t-inherit-mode="extension">
<xpath expr="//button[contains(@class, 'o-calendar-quick-create--edit-btn')]" position="after">
<!-- Add your custom buttons here -->
<button class="btn btn-primary o-calendar-quick-create--create-btn" t-on-click="onCreateBtnClick">
Custom
</button>
</xpath>
</t>
</template>

<t t-inherit="web.CalendarQuickCreate" - mention the name of the module in which the template belongs with template id
here the "web" is the module name and "CalendarQuickCreate" is template id

<xpath expr="//button[contains(@class, 'o-calendar-quick-create--edit-btn')]" inherit the correct button class like this and after
put your button like this

<button class="btn btn-primary o-calendar-quick-create--create-btn" t-on-click="onCreateBtnClick">
Custom
</button>