To insert a watermark in a custom placement, go to your Watermark Settings > Placement > and select 'Custom'  from the drop-down menu:

You will see these custom placement fields:

X Offset: this is the horizontal positioning of your watermark. Use a value of 0 to 1 (to represent percentages). For example, 0 (0%) represents the far left, while 1 (100%) represents the far right, and 0.5 represents the middle (50%). It is recommended to use an integer between 0.01 to 0.99. 

Y Offset: this is the vertical positioning of your watermark. Use a value of 0 to 1 (to represent percentages). For example, 0 (or 0%) represents the bottom, while 1 (100%) represents the top, and 0.5 represents the middle (50%). It is recommended to use an integer between 0.01 to 0.99.

Width: this is the width size of the box where your watermark will be placed inside (the 'box' is invisible). The measurements are in pixels. For example, if you have a longer watermark, you may want to set 300-600 pixels for the width, or for a shorter watermark, you may want to use 50-100 pixels for the width.

Height: this is the height of the box where your watermark will be placed inside (again, this is an 'invisible' box). The measurements are in pixels. For example, if you are using a large-sized font, you may want to use a larger height such as 50-100 pixels, or for a smaller-sized font, use a height 10-25 pixels.

Rotation: this is the rotation of your watermark, represented in angles. 0 = horizontal, 90 = vertical (text runs bottom to top), 45 = angled up to the right, 180 = flipped upside down (yikes, don't use this one!), 270 = vertical (text runs top to bottom)