That light (21w T5) will be nowhere close to satisfying the light requirements of a tomato plant. You can try it, but I think that is a losing battle. You can (maybe) use it to get it started, but you won't get any fruit from it.

Hi, I don't know for sure because I never needed to replace the light bulb but I was always under the impression that it was a T8 light. What makes you think it is a T5?

The link below says that it is T5 and most under-cabinet lights in general are. You can measure the diameter (should be 5/8") to confirm.

Either way, I don't think it is enough light to help a tomato plant grow beyond the seedling well without supplemental sunlight. You can probably start it with that though. Someone with more experience growing tomatoes indoors could probably confirm or disconfirm this.

Here is a link that might be useful: 21w T5