Buy devices in Apple Store - add to Business manager ?

From what I understand if it is an Apple Retail store, they will not be able to do so.  If you have an Apple Authorized Reseller that has a store front, that may be a little different but not 100% sure on that.

Apple Configurator for iOS lets you put retail systems into DEP. 

If you work with your local store's Business team and these devices will get added to DEP, we love it for emergencies or when we have a "Get it now" moment.  If you're still having problems reach out to your Enterprise Apple Rep to make an intro to the store team, and they will also train them on how to link things up if they don't already know how to do it.

Another BIG learning lesson here is DO NOT make your staff other than who needs to know that you can do this in-store and get your corp discount.  I've seen many users finding out they can "ask for the corp discount" and buy themselves a <Insert ABM Item here>, which is now in your Jamf server.  I had a user buy his friends and family devices, and they were very frustrated with it as they couldn't do things others could and always wondered why <Company Name> IT needed to do this or that with the device.  It took us forever to figure out the actual purchase path, so we couldn't just remove the devices as we couldn't figure out where they came from and didn't want to remove our devices by accident.  Luckily we have a friend with access to GSX that could help us track the SN's back as we saw all kinds of unapproved apps etc., and were like, which users doing this???

It finally got resolved once we saw the receipts, and we removed the devices and reminded the user never to do that again.

Thanks for the input.

But overall my issue/question is if going to any random official Apple Store will they be able to do it. Or is it only some Apple stores with special enterprise employees in the back office that can do it. Issue is that this can happen anymore in the world that users may ask if they can go to an apple store to buy an device, so if it is only randomly it will be not easy to give guidelines.
I have heard some that have tried and ask for business manager and the Apple store employees just looked totally lost 

AFAIK this should still be the case and especially now with Apple pushing its new Small Business MDM and other Small Business focuses probably going to find at least one in every store.  I'd say if you have a larger store near you call ahead.  When I worked for Apple many moons ago at least 3 of the stores in Denver had Business Teams full time.

So as I understand the "magic sentence"  when comming to an Apple store is if they have a business team - or else they propably will be a no-go

But let´s say they have a business team and I want to buy a device. Do I just have to inform our company´s DEP number or must it be some of the bunch of apple customer numbers we have  

I think that would be sufficient.  I can ask our Apple Rep for the best way to achieve this in store.  Also, any reason not to keep a few extra of your standard laptops on hand and overnight to a user?  I ask if you have a custom build that's not just an off-the-shelf version, they won't have that in-store.  The other issue this is sticky for some folks is that the user would be buying it on a personal credit card etc.  We found many users didn't want to be on the hook for the interest if they didn't get reimbursed right away.  We, AFAIK, still don't cover interest charges from credit cards used for work matters and only if it is purchased with a company card.