How to check field with pdftk?

I'm trying to fill an institutional pdf (Cerfa n° 12571). All is working except the checkbox.

I have exported fields using pdftk dump_data_fields on the pdf with the checkboxes checked and not checked. For one not checked I get

FieldType: Button
FieldName: topmostSubform[0].Page1[0].Case_à_cocher1[0]
FieldNameAlt: Case_à_cocher1
FieldFlags: 0
FieldValue: 
FieldJustification: Left
FieldStateOption: 1
FieldStateOption: Off

For the checked one I get

FieldType: Button
FieldName: topmostSubform[0].Page1[0].Case_à_cocher1[0]
FieldNameAlt: Case_à_cocher1
FieldFlags: 0
FieldValue: 
FieldValue: 1
FieldJustification: Left
FieldStateOption: 1
FieldStateOption: Off

So the checked one get another line FieldValue: 1. I don't understand what I should put in the value for this field being checked ?

I tried all of this

<</T(topmostSubform[0].Page1[0].Case_&#224;_cocher1[0])/V(Off)>>
<</T(topmostSubform[0].Page1[0].Case_&#224;_cocher1[0])/V(On)>>
<</T(topmostSubform[0].Page1[0].Case_&#224;_cocher1[0])/V(off)>>
<</T(topmostSubform[0].Page1[0].Case_&#224;_cocher1[0])/V(on)>>
<</T(topmostSubform[0].Page1[0].Case_&#224;_cocher1[0])/V(1)>>
<</T(topmostSubform[0].Page1[0].Case_&#224;_cocher1[0])/V(0)>>
<</T(topmostSubform[0].Page1[0].Case_&#224;_cocher1[0])/V(oui)>>
<</T(topmostSubform[0].Page1[0].Case_&#224;_cocher1[0])/V(Oui)>>
<</T(topmostSubform[0].Page1[0].Case_&#224;_cocher1[0])/V(non)>>
<</T(topmostSubform[0].Page1[0].Case_&#224;_cocher1[0])/V(Non)>>
<</T(topmostSubform[0].Page1[0].Case_&#224;_cocher1[0])/V(yes)>>
<</T(topmostSubform[0].Page1[0].Case_&#224;_cocher1[0])/V(Yes)>>
<</T(topmostSubform[0].Page1[0].Case_&#224;_cocher1[0])/V(no)>>
<</T(topmostSubform[0].Page1[0].Case_&#224;_cocher1[0])/V(No)>>
<</T(topmostSubform[0].Page1[0].Case_&#224;_cocher1[0])/V(true)>>
<</T(topmostSubform[0].Page1[0].Case_&#224;_cocher1[0])/V(false)>>

Nothing work !

When I export in fdf with a box checked, I get

<<
/V /1
/T (Case_р_cocher1[0])
>> 

Any idea ?