we want the details field of our work items to have a pre defined text. Since the fields have the attribuded HTML, is it possible to do so?
Yes, you can customize the field to have a default value (Define a default value or copy a value to a field) and you can use html code for the default value:
<FIELD name="Repro Steps" refname="Microsoft.VSTS.TCM.ReproSteps" type="HTML"> <REQUIRED /> <DEFAULT from="value" value="<b>Environment tested:</b><br/>foo<br/><br/><b>Steps:</b><br/><ol><li>&nbsp;</li><li>&nbsp;</li></ol><br/><b>Actual results:</b><br/><br/><b>Expected results:</b><br/>" /></FIELD>