Adding variable to model:
<record id="module_key" model="ir.config_parameter">
<field name="key">name_key</field>
<field name="value">value_key</field>
</record>
loading variable to module:
<template id="assets_backend" name="module_assets" inherit_id="web.assets_backend">
<xpath expr="." position="inside">
<script src="https://url.com/api/js?key=PUT_HERE_THE_VALUE_OF_THE_VARIABLE"></script>
</xpath>
</template>