I don't think you can directly create your own custom placeholders in Scrivener. As far as I know, you can do three things:
1. Use the existing placeholder tags
When you click on
Help > Placeholder Tags List… you can see a list of predefined tags that you can insert into your document and that will get replaced by the appropriate information when you compile your finished document. The information that most of these placeholder tags get replaced by cannot be changed by you (e.g. the current page number), but others are replaced by information that you enter. For example you can use the project title tag as the name of one character, and the author's surname tag as the name of another. You can define five tags under
Project > Meta-Data Settings > Project Properties.
2. Define automatic replacements when you compile
When you compile your final document, you have the option to define character strings and what you want them to get replaced by under
Compile > All Options > Replacements > Project Replacements. Here you can define replacements similar to a normal search & replace.
3. Manually search and replace.
Edit > Find > Project Replace… you can perform a manual search and replace.
Some names are embedded grammatically different than others. For example, if you replace "John" in "John's" by "James", the outcome will not be what you expect. For cases like this, the third, manual option is the safest, as it allows you to find all instances of a name automatically, but authorize replacements on an individual basis.