bank-payment/account_banking_mandate/data/mandate_reference_sequence.xml

<?xml version="1.0" encoding="utf-8" ?>
<flectra noupdate="1">
    <record id="dd_mandate_seq" model="ir.sequence">
        <field name="name">DD Mandate Reference</field>
        <field name="code">account.banking.mandate</field>
        <field name="prefix">BM</field>
        <field name="padding" eval="7" />
        <field name="company_id" eval="False" />
    </record>
</flectra>