Hi,
you need to edit the value of the field in the XML source.
1. Set your text field to rich text
2. Create a text onject with the default value you need as default text and make the portions bold you like.
3. Select the text and then change over to the XML source.
You will see the structure of the text object in a way like this:
4. Select and copy the whole content between the <value></value> tags
5. Now look for the text field in the XML source
It should look like:
6. Replace the existing data between the <value></value> tags with that you've copied before from the text object.
7. Change to the Design view and delete the text object (you don't need it any longer).