Escaping a given character in a string

It is necessary to implement a function that takes a string and a character, and returns a new string in which each occurrence of the specified character is preceded by a backslash, i.e., the character is "escaped". The function should work without modifying the original string.

std::string mark_char(const std::string& s, char ch) {
    // implement
}

Example: Input: "a:b:c", character ':' → Output: "a\:b\:c"