Map payload attributes to destination

Hi,

I uploaded an audience using a CSV file, and also added some other fields as payload attributes. I want to send that audience to a destination (in this case, Marketo Engage). I want to activate the audience and the payload attributes, and from what I read in the documentation, this is possible.

However, when mapping the fields, I'm unable to find any payload attribute. Is there any previous setting to enable payload attributes for activation? 

https://experienceleague.adobe.com/en/docs/experience-platform/segmentation/faq#where-can-i-activate-externally-generated-audiences-to

Thank you!