Select multiple, select other and multiple answers

I want to make a question where the respondent can select multiple answers And other. If they select other, I would like them to be directed to a question where they can specify what they mean by other.

accident_type 1 Cuts and Lacerations
accident_type 2 Bruises
accident_type 3 Minor Dislocations
accident_type 4 Foreign Bodies
accident_type 5 Minor Head Injury
accident_type 6 Minor Trauma to Body including hands / limbs / feet
accident_type 7 Eye Injury
accident_type 8 Other

Welcome to the forum @Ada_S and thanks for adding your profile picture. It helps build community! When you get a chance, please introduce yourself on this forum thread.

What you ask is certainly possible! You'll need to use this syntax:
selected(${accident_type}, '8')

The syntax you're using (${accident_type}='8') will only work when referencing a select_one type question. Check out the section of the XLSForm docs on relevant for more details: