You can purchase additional hashtags to track (IG accounts only) directly from your payment information section here. Simply click "Edit add-ons"
And then select "Add a hashtag". You will then choose which IG account you would like to add your additional hashtag(s) to. (which will bring you to the payment screen to complete your purchase.)
Additional hashtags are billed at $9/addtional hashtag per month (or $81/addtional hashtag yearly)