In just about every case that comes to mind, you would use a definite or indefinite article when describing your employment. The reason for this is that the job title (such as "Engineer", "Farmer" or "Accountant") is a noun and should be treated as such in a sentence.
In this situation, the correct sentence would be "I am a Senior IT Engineer" (one of many Senior IT Engineers) or "I am the Senior IT Engineer" (you are the only Senior IT Engineer).
In the same way that you wouldn't say "I am person" but rather "I am a person", saying "I am Senior IT Engineer" is incorrect.