As an example, have a look at my profile on D.O., which contains this link.
So make sure you have a similar Organization created there, which you can do via Create Organization, which is the link you can also find on Organizations.
If you're not the current owner (or something like that) of such organization, then you need to think of a way to get that Organization created around there. I don't remember any validation (when I registered the one I refer to), but what I would do:
- Check with the organization you want to refer to if they are OK with getting such registration.
- Have them complete such registration, or ask permission that you do it for them (and have them QA review it afterwards if needed).
If it's a huge organization, it may be a challenge to get approval from such organization, eg to find the right person to ask, and then get the actual approval. In that case, as an alternative, maybe you create something anyway about that organization, but without mentioning their actual name, and only using information that is considered as not confidential? But keep in mind that some companies may not want the world to know that they use Drupal ... eg for competitive reasons ...
Extra hint (for future occasions): try to make it part of contract negociations, something that they confirm that they are aware (and will not object) that you will register them as a company (if they are not yet registered). Possibly adding an appropriate annex to your contract with the details of that registration. It'll slow down contract negotiations, but that's probably the best moment to get approval. If needed offer them some kind of reduction on your rate to make them accept it. And of course, also explain the added value to them for being listed there (like free advertising?) ...
Note: there is also a Marketplace FAQ which may give you more ideas to get an organization registered correctly.