To create an SPF record, log in to your customer account. Click on Nameserver in the left side menu of the customer area and afterwards on the little eye icon to modify the respective domain. Then follow the subsequent steps:
To add a SPF entry, leave the Name field empty!
Choose Type TXT from the drop-down menu.
Enter the SPF entry into the Value field.
Information about the syntax can be found here: https://www.spf-record.com/generator
Thereafter, the SPF entry is set up correctly and will be active within a few hours.