While users cannot save a draft of an ad, there is a workaround:

Sometimes users wish to save a draft because they're not ready for the ad to go live, or need a second set of eyes to confirm the ad before launching.

While you cannot save a draft in StreetText, the best steps would be to launch an ad, then head to the ads tab and pause the ad.

This ad cannot be edited once launched, but offers an opportunity for users to prepare ads in advance in this manner.