For 'Just a moment' the indefinite article is correct to use in this case. For polite requests like this we use the indefinite: 'Give me a chance.' 'I need a moment/minute/second to catch my breath.' To use 'the moment/minute/second' is more likely to be used when recalling something from the past or something of great significance in the future: 'I still remember the moment I met you.' 'The second I walked in to the room, I was terrified.' 'You have to call and tell me the second you find out what happened.' Using the definite article when referring to a moment/minute/second is usually done when it has great significance; in the case of a canned expression like 'Just a moment' it's not an important, significance moment so we then use it with the indefinite article.