The pdfFormField is a handlebars helper, so you need to proceed in the same way as when calling other helpers.

Filling attribute value from context using path

You shouldn't put the path to the brackets {{}} when filling helpers parameter. It should look like value=myData

Demo
https://playground.jsreport.net/w/anon/XgVqQLNT

Calling extra helper to fill the param

You need to use subexpression to call another handlebars helper as attribute param. It should look like value=(myHelper)

Demo
https://playground.jsreport.net/w/anon/SMSHyEw3

Documentation
https://handlebarsjs.com/guide/expressions.html#subexpressions