Sendy last name custom field

Hi, awesome plugin! I’ve setup user sync with Sendy and it’s working very well. The only issue I have is that the first name and last name fields merge when they’re sent to Sendy and I’m not able to use only their first name in emails. I’ve created the last name field in Sendy and I’ve mapped first and last separately but when it goes into Sendy it shows the last name field empty.

Is there any way I could make this work? Thank you ??