Stir 1 tbsp sugar into lukewarm water in a small bowl. Sprinkle the yeast over the top. Leave until foamy, about 5 minutes. Stir to dissolve yeast.
Stir potato, milk, remaining sugar, the butter and salt in a small saucepan. Heat just to warm the mixture and melt butter. Using a rubber spatula, push mixture into a large bowl through a sieve to remove any lumps in the potato. Stir in yeast mixture and 3 cups (420 g) flour to form a dough.
Sprinkle remaining flour on work surface. Turn out dough and knead, working in flour, until smooth and elastic, about 10 minutes. Add more flour as needed to prevent sticking. Place dough in a lightly oiled bowl. Turn to coat. Cover loosely with plastic wrap. Leave to rise in a warm place until doubled in volume, about 1 hour 15 minutes.
Lightly oil a 23 x 13 cm loaf pan. Punch dough down. Knead briefly. Press into loaf pan. Cover loosely with plastic wrap. Leave to rise in a warm place until doubled in volume, about 45 minutes.
Heat oven to 180°C. Brush top of loaf with glaze. Sprinkle with poppy seeds. Using a serrated knife, cut a 1 cm-deep slit lengthways down the middle of the loaf.
Bake until browned and loaf has pulled away slightly from edges of pan, 40 to 45 minutes. Turn out onto a wire rack. Cool for at least 1 hour before slicing.