You can create a basic cross shape in Affinity Photo via a rectangle. Duplicate the rectangle by holding down the alt / option key and rotate by 90 degrees. You may wish to re-size the width or height of one of the bars as well as position it a third of the way up the first. You can then combine them via the layers menu.
There is no tool but you can use the square star tool instead. Use the sides at 4 and cutout of about 85%. You can use the interactive dot to modify the cutout and create thin or thick plus designs. You can fill it with solid color, no color or gradient etc. You can modify the design up until the point you rasterize the layer or go to the convert to curves command / button.