I just had prepared this. The screenshot are from business central on premise but it should work similar in online.
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How do I create a simple CSV payment export using the data exchange definition?
Step 1 – Create a new Data Exchange definition. Mine is called “CSV”
Make sure it is Type Payment Export.
Reading/Writing Codeunti set to 1276, Reading/Writing XML Port to 1230 and Ex. Data Handling Codeunit to 1277.
For testing my file will get 4 fields so I put those in the column definition and in the line definition, I set the number of columns to 4

Next is to set the field mapping. Important is here that we need to map to table 1226. And I map my fields to field from this table.

That’s it for the data exchange definition, now we need to be able to use it and for that we need a Bank Export/Import Setup.
Make sure you use the Processing Codeunit 1206.

Then we need a payment method. We need it in case we have multiple line definitions. In our case we just have one so we can set it to “DEFAULT”.

Now we setup the bank so it knows how to use our export. I will use GIRO in this example. Set the Payment Export Format to CSV and make sure you also have set a Credit Transfer Msg. Nos.
In the journal Batch make sure the bank is selected as balance account and that you allow payment export.

Now in the payment journal add some data or do a payment suggestion. Make sure to use the bank we did setup and set the type to electronic payment. I also filtered to vendor 10000 to have less entries.

Select a recipient bank account and set the payment method to what we have prepared:

Now click “Export” and you will get the CSV file:

Hope that helps.