Just found out that as of July 2018 retired correctional officers are eligible state deduction of 15000 dollars, how do i applied this to my state tax form?

After you enter the 1099-R you will indicate this on the follow up screens and it will be referenced in the state interview if any other entries are needed.  

Have you already filed and need to amend your state return now?

 You may  qualify for a pension exclusion of up to $15,000, on the MD state return, if you meet the following requirements: 

1. You were 55 or over on the last day of the tax year, AND
2. You were not 65 or older, or totally disabled, or have a spouse who is totally disabled, AND
3. You included on your federal return taxable income received as a pension, annuity or endowment from an “employee retirement system.” Please note that these include qualified defined benefit and defined contribution pension plans, 401(a) plans, 401(k) plans, 403(b) plans, and 457(b) plans of the Internal Revenue Code, AND
4. The retirement income is attributable to your service as correctional officer, a law enforcement officer or fire, rescue, or emergency services personnel of the United States, the State of Maryland, or a political subdivision of Maryland.

Reference: https://taxes.marylandtaxes.gov/Individual_Taxes/Individual_Tax_Types/Income_Tax/Filing_Information/If_You_Are_a_Senior_Citizen/Maryland_Pension_Exclusion.shtml

As others have indicated, you will be asked if your qualify in the state interview portion of TurboTax. It is entered as a deduction on state form 502SU. See https://ttlc.intuit.com/community/state-taxes/discussion/how-do-i-claim-the-maryland-hometown-heroes-tax-credit/00/433662

Retired corrections officers are indeed eligible for a $15,000 exclusion in Maryland on their pension under the conditions described above. However, rather than being reported on form 502SU (where most other subtractions are reported), this particular pension exclusion appears on line 10b on form 502. Note that this exclusion comes from Pension Computation Worksheet 13E on page 10 at https://forms.marylandtaxes.gov/18_forms/Resident_Booklet.pdf.

See the instructions for "10b. PENSION EXCLUSION FOR RETIRED CORRECTIONAL OFFICER, LAW ENFORCEMENT OFFICER, OR FIRE, RESCUE, OR EMERGENCY SERVICES PERSONNEL" on page 7 at the link above.

The answer at the link that Hal_Al referred to appeared to have been written before the actual instructions for this exclusion were written by the state (if the law was changed in July 2018, it obviously took time for the state department of revenue to redesign the forms and write the instructions and make them available to tax software companies like Intuit). In the event, how the state treated these pensions changed and the current instructions can be found at the link above and at https://taxes.marylandtaxes.gov/Individual_Taxes/Individual_Tax_Types/Income_Tax/Filing_Information/If_You_Are_a_Senior_Citizen/Maryland_Pension_Exclusion.shtml.