Securing an endorsement, especially if it's a "cold call" on your part, is going to take some work from you. Your goal in the process is to sell your community. How do you do so?
Have a community worth advertising
The first step in all of this, is to have a community you are proud of and can show off. Make a list of what sets your community apart from others in your field. What makes your community better than a more well known one? Don't be afraid to provide examples showing off your community too. If you aren't proud of what your community is and has done, why should someone else be concerned with even looking at it?
Personalize the communication
The down side of a cold call is that you probably don't know the person you are communicating with at all. Do a bit of research ahead of time. Find something about them (in a non-stalker way) that you have in common. It's much easier to form a relationship with someone if you have something in common. This relationship will be short and professional, but it will still exist. If you are remembered as the person that likes the same thing they do or the community that supports the same cause as them, you'll stick in their mind just a bit longer and they may be willing to spend a few minutes either talking to you or visiting your community.
Ask for the endorsement
Come out and ask for the endorsement. Don't beat around the bush. Make it clear what you are asking for and make the request short. Your email - I'm assuming you are communicating via email - shouldn't be a novel (or even a chapter in a novel). It should be a short request explaining what you are looking for and why you think the editor should consider, at least visiting the community. Part of this email should provide information on how to join/interact with the community and perhaps a link or two to one of the items on your list from above. Explain what you are looking for - a few sentences of support in an upcoming journal. You aren't asking for an article. Your goal is to minimize the amount of extra work they have to do. A few sentences is much easier than an entire article.
Focus on getting another endorsement first
If your end goal is to get an endorsement from this journal, it may be worthwhile to put it on the back burner for a while and get the support of someone else first. Utilize one endorsement to get another. When you finally reach out to the journal, you can then say that your community has been supported by others (and name names). Those other endorsements show that you are known in the field and have the support of others in the field. This is provides another reason for the journal editor to take those few minutes of of their day and check out your community. Leverage your community in determining who these earlier endorsements could be. These endorsements will also help to show that you aren't a random promoter. Going this route does slow down your path to getting the endorsement of the journal, but it helps you build a network of supporters.
Don't fear rejection
You may not get the endorsement you seek. In that case, remain professional. Thank the editor for their consideration, move on and don't pester them. Move on to finding other possible endorsements and other possible ways of spreading the word about your community. If you are lucky, you received a reason about why the endorsement can't be given. Focus on fixing that reason (if you and your community feel it needs to be fixed). In time, you can revisit the desire to get their endorsement.