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Found solution for this :

var htmlHeader = token_form.addField({
	                 id: 'custpage_header',
	                 type: ui.FieldType.INLINEHTML,
	                 label: ' '
	          }).updateLayoutType({
	                 layoutType: ui.FieldLayoutType.OUTSIDEABOVE
	          }).updateBreakType({
	                 breakType: ui.FieldBreakType.STARTROW
	          }).defaultValue = "text"

Having control over layout is one reason I recommend reaching for a SPA design rather than the traditional native Suitelets approach. Not sure if that's an option for you or not but thought I'd mention.