if MFS in 2018 use your spouse's AGI only for your spouse. if you didn't file in 2018 use 0 for yours or if you did file use the amount from your 2018 1040 line 7
don't know why you chose MFS in 2018. it was not necessary if the only factor was that only one spouse had income. there is no requirement for both spouses to have income to file a joint return. you can amend 2018 to switch to a joint return
here are the steps if you decide to amend
https://ttlc.intuit.com/community/amending/help/how-do-i-amend-married-filing-separately-returns-to-married-filing-jointly/00/25715