From adding text to images to inserting objects that weren't present at the time of shooting, knowing how to set up a placeholder in a photo is an important skill. You can approach the problem either using software or simple practical photography hacks. By creating a process, you can also simplify how efficient your overall workflow will be.
In Photoshop, you should be able to mask out an area you want as a placeholder using the various selection tools. If you're extremely lucky with your initial photo, you might be able to get away with masking out the area using the rectangular or polygon tools and then filling them. For more complex regions of images, the quick select and magic wand tools are also useful. Be aware that you may need to add a degree of feathering to the selection in order to prevent overly harsh lines from appearing in the image.
Use an Actual Object
This is an especially popular choice among those creating stock images, as there's a good chance the end user will likely want to add some text to a specific area. Something as simple as handing a model a white placard often does the job very well. Just be prepared to some day see someone on the internet take advantage of that space to do something unexpected with images you created.
If you're looking to produce a placeholder in a more complex space, you can resort to variants of the old school green-screen techniques that are popular in the movie production and video game industries. These can then be edited out later in Photoshop. It's also possible to utilize actual objects for placeholders, such as taking a ball and painting it completely white to mark out the desired area.
You may have to adjust your white balance and color levels during the initial shooting process. It's wise to shoot in RAW mode in order to improve your ability to make adjustments during post-production.
If you're looking to add something into an image with a placeholder, you may want to invest in plain backdrops to make the process simpler. Shooting with a backdrop will permit you to separate your subject in post-production, speeding up the whole process. You should try to make good notes about lighting during both shoots to ensure that everything lines up properly between the two images you'll be compositing.