Save a field when creating a model

Hello, I have a

casts

casts

casts

casts field in my model and I want to save two other fields into the particular field.
This question is similar ❓┊helpCustom value for field (fill and save field with custom table column) but the provided solution does not work for me while using

mutateFormDataBeforeCreate

mutateFormDataBeforeCreate

The field I want I can set it inside

mutateFormDataBeforeCreate

mutateFormDataBeforeCreate

but it fails

SQLSTATE[HY000]: General error: 1364 Field 'myField' doesn't have a default value

SQLSTATE[HY000]: General error: 1364 Field 'myField' doesn't have a default value

SQLSTATE[HY000]: General error: 1364 Field 'myField' doesn't have a default value

SQLSTATE[HY000]: General error: 1364 Field 'myField' doesn't have a default value

.
How can I save into this field on creation?
Thank you.