Preheat your oven to 350 degrees F. Line a 9 x 5'' loaf pan with parchment paper (cut to fit the length of pan) and leave the overhang on the long sides of the pan. Spray the insides of pan with cooking spray and set aside.
In a medium bowl, sift together the flour, baking powder and salt. Set aside.
In a small bowl, combine the sugar and grapefruit zest. Use your fingers to rub the zest into the sugar until the mixture is fragrant, about one minute. Set aside.
In a large bowl, combine the eggs, oil, sugar + zest mixture, and vanilla. Beat with a handheld mixer on high speed until the mixture is light and thick, about 4 minutes. Reduce the speed to low and add in half of your flour mixture. Mix until just combined, then add the yogurt, mixing again on low until just combined. Add the remaining half of your flour mixture, followed by 5 tablespoons of the grapefruit juice and 1 tablespoon of the poppy seeds. Mix until just combined, then use a rubber spatula to gently fold in anything that may remain on the sides of your bowl.
Pour the batter into your prepared loaf pan and give it a slight shake to settle the batter into the corners of your pan. Bake for 45-60 minutes until the cake is set and a toothpick comes out clean when inserted into the center of the loaf.
Remove the finished loaf from the oven and set the pan on a wire rack for 15 minutes. Remove the loaf from the pan and take carefully remove the parchment paper. Cool the loaf completely on a wire rack before glazing.
To glaze the cool loaf, combine the powdered sugar, the remaining 1 tablespoon of yogurt and 1 tablespoon of grapefruit juice in a small bowl. Whisk until smooth, then spoon the glaze over the loaf.
Store the loaf in an airtight container at room temperature for up to 3 days or in the refrigerator for up to 1 week.