Make sure you have HTML Asset File and Tracker Asset as they are required.
Create Creative by using Create Creative API.

Creative Sizes

For each creative size you need to create separate creative by using same asset file.
Properties related to creative size are: width , heightand rewarded_type, they influence the result Ad Format:

If rewarded_typeisrewardedthen the Ad Format is Rewarded
Otherwise, if width>heightandheight <= 250px then Ad Format is Banner
Otherwise, Ad Format is Interstitial

Creative Size Name	`width`	`height`	`rewarded_type`	Ad Format
Landscape_Mobile_Phone	480	320	normal	Interstitial
Landscape_Tablet	1024	768	normal	Interstitial
Portrait_Mobile_Phone	320	480	normal	Interstitial
Portrait_Tablet	768	1024	normal	Interstitial
Landscape_Mobile_Phone	480	320	rewarded	Rewarded
Landscape_Tablet	1024	768	rewarded	Rewarded
Portrait_Mobile_Phone	320	480	rewarded	Rewarded
Portrait_Tablet	768	1024	rewarded	Rewarded
Mobile_Phone	300	50	normal	Banner
Mobile_Phone	320	50	normal	Banner
Mobile_Phone	320	100	normal	Banner
Tablet	728	90	normal	Banner
Tablet	468	60	normal	Banner
MRec	300	250	normal	Banner

Relevant parameters

Following are the relevant and required parameters to use with Create Creative API:

name
tracker_id
type: must be javascript
asset_file_id
width
height
rewarded_type
mraid_type

Additional optional parameters:

start_time
end_time
creative_attributes
ad_gap_id
ad_gap_reporting_metric