Primary keys and foreign key are for relationship between two tables and should have nothing to do with running a query on a sql table. It should not be part of the table name and you can have more than one key field per table. I would not have table in my field name either. I would change my notation some, but it would not be leaving the underscore out.
Inclose such name in double quotes such as:
"SELECT * FROM YourTable WHERE YourTable.\"_fkForeignKey\" = 5"
When this string is evaluated, \" will be replaced with the " character, but without FileMaker interpreting it as the end of a literal string of quoted text.