The configuration you're looking for can be found in the User-defined Defaults. To modify these entries you will first need to enable the Developer mode (https://www.odoo.com/documentation/18.0/applications/general/developer_mode.html) and then navigate to Settings -> Technical --> Actions -> User-defined Defaults.

In here you will find records that pre-define the values (Default Value (JSON format)) of certain fields in Odoo - two of these fields happens to be Account Receivable (Contact) and Account Payable (Contact). You may find multiple records for these field in a multi-company setup, so ensure you modify the ones relevant to you (or create additional records for specific companies if needed). 

Now, since this is a generic view, you can not simply select the record you want for Default Value (JSON format). Instead, you need to find the ID (not the (numeric) Code!) of the Account you want to have assigned to new Customers by default. To do so, open the form view of that Account (i.e. Accounting (or Invoicing) -> Configuration --> Accounting -> Chart of Accounts and click the View link at the relevant line. Your browser will redirect you to a page similar to this: https://your-odoo-domain.odoo.com/odoo/action-266/241. Here, the last numeric value, 241 in this example, is what you want to set as a Default Value (JSON format) to the User-defined Defaults record.

Note: Already existing Customers need to be modified manually (or by doing an update-import).