You need to be at least Jr. Member to be able to wear signature but usually bounty program only accept Member level to participate in their signature campaign

Rank			Signature styling			Merit requirement
Newbie			No styling (including links) allowed. Max 50 characters.			0
Jr. Member			Max 150 characters.			0
Member			Links allowed. Unlimited length.			10
Full Member			Color allowed.			100
Sr. Member			Size allowed			250
Hero Member			Background color allowed			500
Legendary			Everything			1000