Hello @masanortakamoto ,
please follow these steps:
1. Initialize an Array variable:

2. Generate an "Apply to each" control: for each Multiple choice column value, append the Array variable using the following JSON:
{
"Value": "@{items('Apply_to_each')?['Value']}"
}

3. Then, you'll be able to add the variable in the related field:

Please let me know if the issue is fixed.
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BR,
Marco