To do this, you must exalt the 4 main cities of the alliance, which is possible by doing a quest (it is better to do the quests of each city, for example, stormwind in the same land to exalt that land)
After you have exalted the 4 main cities of the alliance, you
must first press c (there is a c on the keyboard !!)
Then at the top of the opened page, there is a section called title, which by clicking on it and changing it to ambassador, you can be an ambassador. Become !!
1- This explanation is not only about alliances, hordes can also do this by exalting the 4 main cities.
2- Apart from ambassador, there are other phrases, for example: argent champion
3- In the cataclysm patch (4.3.4), you must exalt 5 cities
4- You can see these phrases on your hero’s head while playing (for example, ambassador kido )