Immediate Coverage Start Date Based on Employee Selection

I have a situation for which I have not yet found an appropriate solution.

During Self-Service Benefits enrollment, some employees are eligible to start plan coverage immediately (coverage start date = date of enrollment). However, this immediate coverage must be optional — meaning the employee needs to indicate that they want the coverage to begin on the same date as the enrollment.

For other employees, the rule is different: the coverage start date will be defined automatically (for example, the first day of the following month), and this will be handled through an eligibility formula.

The challenge is to allow only certain employees to opt for immediate coverage, in a simple and transparent way during the enrollment process.