I pay rent to my parents and am over 18 -- can I claim this amount and enter in the WI state tax form when filing my taxes? They are not claiming me on their taxes.

It depends on whether you meet all the conditions.  The qualifications are shown below for the Wisconsin (WI) Homestead Credit.  Not being claimed on another return is only one of the qualifier.

Do I qualify for homestead credit?

To qualify for homestead credit for 2017 you must:

• Be a legal resident of Wisconsin for all of 2017, from January 1 through December 31.
• Be 18 years of age or older on December 31, 2017.
• Have less than $24,680 in household income for 2017. (See the definition of "household income".)
• Meet one of the following conditions (new for the 2017 claims filed in 2018:(
• You (or your spouse, if married) have earned income during the year.
• You (or your spouse, if married) are disabled. (See definition of "disabled".)
• You (or your spouse, if married) are 62 years of age or older at the end of 2017.
• Own or rent your Wisconsin homestead that is subject to Wisconsin property taxes during 2017. (Persons who reside in mobile or manufactured homes or nursing homes that are subject to property taxes may also qualify to file a claim. See the definition of "homestead".)
• Not be claimed as a dependent on someone else's 2017 federal income tax return. (Note: This limitation does not apply if you were 62 years of age or over on December 31, 2017.)
• Not have lived for the entire year 2017 in housing that is exempt from property taxes. (Note: Property owned by a municipal housing authority is not considered tax-exempt for homestead credit purposes if that authority makes payments in place of property taxes to the city or town in which it is located. If you live in public housing, you should check with your rental manager.)
• Not, at the time of filing a claim, be living in a nursing home and receiving Title XIX medical assistance.
• Not be claiming Wisconsin farmland preservation credit for 2017.
• Not be claiming the veterans and surviving spouses property tax credit on the 2017 real estate taxes.
• Only one claim may be filed per household (married couple residing together). See the definition of "household."
• Not be filing a claim on behalf of a person after his or her death.
• Not have received Wisconsin Works (W2) payments of any amount or county relief payments of $400 or more for each month of 2017.

  Note: If you received any amount of a Wisconsin Works (W2) payment in 2017 or county relief payments of $400 or more for any month in 2017, your property taxes and rent have to be reduced by one-twelfth for each month you received any of these benefits.