How do you add a Custom Field to a Single Doctype?

Hi, I want to add a custom field to the doctype “School Settings”. But the “School Settings” doctype does not come up in the document drop down list when on the Setup>CustomField form, please help.

I checked it. The school setting is not directly accessible in Customize Form.
What is your use case?

So, first, go to School Setting form then select Customize option from Menu button. Add your custom field.
Check following screen shot

thank you, that works right. I want to add more settings for a school. eg whether school is primary, tertiary or university, a choice that will determine which customisations will apply as their setups are different.

@Sangram once I add a custom field in the single doctype, then if i use bench export-fixtures command this custom field is not added in the custom_field.json file which is available in the custom app’s fixtures folder, any solution thanks in advance
@rmehta @JayRam