Here is the complete reference guide:

http://www.debenu.com/docs/pdf_library_reference/FunctionGroups.php

Here is the complete section regarding form fields:

http://www.debenu.com/docs/pdf_library_reference/FormFields.php

In the online developer guide there's a section regarding your form field issue:

http://www.debenu.com/docs/pdf_library_developer_guide/debenu_quick_pdf_library_10_developer_guide.pdf

With (first: AddTrueTypeFont) and then SetFormFieldFont you can use your added (before!) fonts with - for example bold and underline style:

http://www.debenu.com/docs/pdf_library_reference/AddTrueTypeFont.php

http://www.debenu.com/docs/pdf_library_reference/SetFormFieldFont.php

If your main issue is how to get a bold font try this SetFormFieldStandardFont:

http://www.debenu.com/docs/pdf_library_reference/SetFormFieldStandardFont.php

With SetFormFieldRichTextString there are still more properties dealing with fonts:

http://www.debenu.com/docs/pdf_library_reference/SetFormFieldRichTextString.php

For this you need the xml-string generated by - for example - acrobat.

If you wanna use this option here's a helpful topic from the community:

http://www.quickpdf.org/forum/populate-rich-text-form-field-with-formatted-text_topic2834.html