Remember that you must be covered by insurance that qualifies as Minimum Essential Coverage (MEC) to receive QSEHRA reimbursements.
As long as your parents' plan qualifies as MEC, then you are eligible to participate in the QSEHRA. However, you cannot claim reimbursement for your insurance premiums. If your QSEHRA is set up for premium-only reimbursement, you will not be able to take advantage of the QSEHRA.
If your QSEHRA allows for the reimbursement of medical expenses, then you can use your entire allowance to claim reimbursement for medical expenses.