Hi Dongzix,
Sorry for the delay in response.
I have checked the same document at my end.
In this form, the fields that you have to fill are already highlighted. The signature field is not highlighted by default.
So, you will not be able to add the signature here.
As you are filling the form online, once you complete all the highlighted field click on Validate tab.
Once the form validation is completed, you will get the pop-up window as shown below:

It is mentioned there " You will be asked to provide electronic signature once you are ready to transmit your documents to CIC."
Check if that helps.
For more information regarding the form, I would suggest you to get help from there Help centre.
Let us know if you need any help.
Regards,
Meenakshi