This article explains how to set a default payment method on Metal Pay.
Step 1: Tap your profile picture to open settings.
Step 2: Select 'Payment methods.'
Step 3: Choose a funding source. Note: Only debit and credit cards are accepted.
Step 4: Tap 'Set default' to make it your default method.
When a purple checkmark appears next to your chosen method, it has been set as default!