How do you adjust the 'margins' in the Premiere Pro program window?
Possibility #1: If there's not just black filled rectangles from the rectangle tool around your text, then it looks like you might have nested your text into a sequence with smaller dimensions. Find the text on your timeline and check if it is in a nest. If so, try:
1. Right-click on the nest in the timeline
2. Choose 'Reveal in Project' to find the nest in the Project panel
3. Right-click on the nest in the Project panel
4. Choose 'Sequence Settings'
5. Change the Video > Frame Size 'horizontal' and 'vertical' values to match the sequence you added the nested text to.

Possibility #2:
Another way you can achieve this effect is in the Effects controls panel for the text clip. If you were to scale the video down, it would have hard borders. Check that the video scale is at 100%.