Hi everyone!
Just wanted to share how I managed to add the "Print" button for a custom model in Odoo 15. Hopefully this helps someone!
 
Report Template (QWeb) ​

Here’s an example of a simple report template:

<template id="my_model_report_template">
    <t t-call="web.html_container">
        <t t-foreach="docs" t-as="doc">
            <div class="page">
                <h2>My Model Report</h2>
                <p><strong>Name:</strong> <t t-esc="doc.name"/></p>
                <p><strong>Description:</strong> <t t-esc="doc.description"/></p>
            </div>
        </t>
    </t>
</template>

Report Action
To make the report available and add the "Print" button directly in the form view of your custom model, define the report like this:  

<record id="action_my_model_report" model="ir.actions.report">
    <field name="name">My Model Report</field>
    <field name="model">my.model</field> 
    <field name="report_type">qweb-pdf</field>
    <field name="report_name">my_module.my_model_report_template</field> 
    <field name="print_report_name">'My Model Report - %s' % (object.name)</field> 
    <field name="binding_model_id" ref="model_my_model"/> <!-- This adds the "Print" button -->
</record>

Hope this helps! 

Best regards!

hi Niyas,

does this method works in odoo 11?