Here's a cheat sheet I made with the steps to put text behind an object to be motion tracked. The check boxes are very confusing so I hope this can help someone.
@Roger Bansemer : Thank you for the cheat sheet :-) Tried it already : 
In HDR : you must turn off HDR in the preview window to be able to see the motion tracking area.
Glad it was helpful. I can't tell you how long it took to figure out those few check boxes in order for it to work properly.
A bit clumsy that the normal Bézier area has no free form like in Premiere, now you have to hope the object you want to eliminate is rather square. Also strange that the Bézier area of the motion tracking function is more flexible, it doesn't need 90° corners, otherwise with the camera on a tripod there would be no need for motion tracking a building...
Anyway thanks again, even with your help it was quite complicated : didn't see anything until I disabled HDR.
I don't think you are actually limited to a square- change the bezier mask shape to curve and draw points to match whatever shape you want. Then uncheck edit.
Here's a very irregular mask (hit invert if you're trying to eliminate it)
You're right. Using the "curve" you can mask around irregular objects. Here's a youtube video which is quite good except he goes so fast at a few points that no matter how many times you watch it, you won't catch some of the check boxes that need to be checked in order for it to work.
Thank you for your work.
Clear step by step instructions, easy to follow. Amazing that an illustrated guide can become more understandable than a video tutorial.
Here's a cheat sheet I made with the steps to put text behind an object to be motion tracked. The check boxes are very confusing so I hope this can help someone.
@Roger Bansemer You can also use the Blend mode in General options to see what your masking with the curve
It would be useful to be able to zoom in on a selected part of the preview window. I am using a 65 inch tv to preview, and even then it is hard to mark some fine objects perfectly, in "curve" mode the tiny yellow handles on each point are difficult to grab. If you watch closely you see that the text is in front of the flagpole on the right of the tower. Not that anyone will notice when they only see it once...
An example with 3 Bézier areas. By now you must know I live in Bruges 😉 
@Bruggeling 👍 I've been asking for zoom on the preview & better control +'s for a while,
32" 4k screen,
In this pic there's 2 +'s for the added gradient, the top ones easier to see but the bottom one is lost,
Screen grabbed & zoomed in it's still hard to see
I have to lower the opacity level of the media sometimes to find these tiny tiny grey opaque +'s 😒
@Former user Dear oh dear, difficult to work like that !
The points I mentioned on the Bézier control in "curve" mode have 2 extra points on the side, to curve the line, problem is they are in the same colour as the centre point (yellow), and are virtually impossible to grab with the mouse, so they are pretty useless.
@Bruggeling Yep i know which you mean 👍 tiny tiny 😒🤦♂️🧐