Any chance you can use the Numeric Entry widget instead? Because it was made for that. It comes with a title and a tooltip.
Otherwise you can use a TextField with attribute HintText to indicate the user needs to enter a number, plus validation as explained here: https://support.ptc.com/help/thingworx/platform/r9.6/en/#page/ThingWorx/Help/Mashup_Builder/Widgets/ApplyingValidationRulesToWidgets.html
You would bind the Validate event to an expression of type "Validator", add a text parameter and this code:
if (text.match(/[^\d]+/g))
output="invalid";
else
output="valid";
Probably add a plus/minus to the regex if negative ints are acceptable.