I help run a Facebook group called Surface Pattern Design Critique Group. I put together a few suggestions on how to ask for a critique. The group is open for any designer looking to get feedback on their work so they can grow.
It can be difficult knowing exactly what to write when asking for a critique. Here's a quick rundown of how to do it.
First, look at your artwork objectively. Identify what's working for you and what isn't. You're asking for a critique, so there's something about your design that you're unhappy with. What's giving you the most trouble in the design? Look at your artwork from an objective pov and play the devil's advocate. Creating artwork for personal enjoyment is just as a legitimate reason to create artwork, but creating commercially viable artwork means that your artwork has to stand on its own without you to hold its hand. You're attempting to sell this artwork to other people who do not have the same emotional ties as you the artist and will judge it based on their own tastes and by market trends.
Second, it's not the critiquer's job to finish your design for you. Critiques are to help you improve your design or to help you check if you've overlooked something. Staring at a design for too long will ultimately lead to design blindness. So take a break before asking for a critique. Go do something away from the computer, watercolors, or paintbrush and give your eyes a break. Then come back and look at your design with fresh eyes and see if you can identify your problem areas so you can better ask for a critique.
Third, identify what you like about the artwork and what you don't like. It's fine if you can't put your finger on why exactly your design isn't working quite right, but take responsibility for your work. You made certain choices in your design process, so own them. It's also perfectly fine to show two different versions or colorways of the same design and ask which one is better, but again, it's not the critiquer's job to do your work for you. Figure out before posting what works and what doesn't work about each option. Even if you are a total newbie and don't understand the mechanics of your design's problems, you should still be able to figure out where your problem areas are—otherwise, why are you asking for a critique?
Finally, remember that you are asking other artists to devote time and energy to look at and analyze your design. Please be respectful of this and put some effort into asking for a critique. Ultimately this is a benefit to you as an artist because learning how to navigate around your problematic areas when creating new artwork will help you grow and develop your skills as an artist even more.