split variable into command option arguments in sh

It's easier with zsh:

From your scalar variable containing a ,-separated list:

DOMAINS="domain1.tld,domain2.tld"

split on , into an array:

domains=( ${(s[,])DOMAINS} )

zip a opt=( -d ) array with it:

opt=( -d )
./example-script.sh ${opt:^^domains}

If ./example-script.sh supports -ddomain1.tld in addition to -d domain1.tld, you can skip the intermediary array and use:

./example-script.sh -d$^domains

In bash, you could use a loop:

IFS=, # split on ,
set -o noglob # disable glob
domains=( $DOMAINS ) # split+glob
args=()
for domain in "${domains[@]}"; do
  args+=( -d "$domain" )
done
./example-script.sh "${args[@]}"

Or you could generate code in the shell syntax with:

IFS=, # split on ,
set -o noglob # disable glob
domains=( $DOMAINS ) # split+glob
if [ "${#domains[@]}" -gt 0 ]; then
  printf -v args ' -d %q' "${domains[@]}"
else
  args=
fi
shell_code="./example-script.sh$args"
eval -- "$shell_code"

If ./example-script.sh supports -ddomain1.tld in addition to -d domain1.tld, you could also do:

IFS=, # split on ,
set -o noglob # disable glob
domains=( $DOMAINS ) # split+glob
./example-script.sh "${domains[@]/#-d}"

Where we prepend -d to each element of the $domains array.

Standard sh doesn't have arrays nor read -A/-a and standard printf doesn't have %q (and the implementations that support it as an extension don't always generate sh-compatible quoting), but you could use the first bash approach above and "$@" in place of an array:

IFS=, # split on ,
set -o noglob # disable glob
set --
for domain in $DOMAINS; do # split+glob
  set -- "$@" -d "$domain"
done
./example-script.sh "$@"

Another option is to call awk to do the splitting and generate sh-compatible code:

eval "./example-script.sh$(
  LC_ALL=C awk -v q="'" '
    function shquote(s) {
      gsub(q, q "\\\\" q q, s)
      return q s q
    }
    BEGIN {
      n = split(ENVIRON["DOMAINS"], domains, ",")
      for (i = 1; i <= n; i++)
        printf " -d %s", shquote(domains[i])
    }'
)"