How to apply my signature in the form before I send it to someone else for their signature?

It's not possible to set a default signature in the eSignature widget(s) so the workaround would be the following:

a. Save and Continue (session URL method).

1. Add a pagebreak in the form: https://www.jotform.com/help/119-How-to-Create-Multi-Page-Forms.

2. Add the agreement on the first page so you can sign it.

3. Build the session URL for each client. For example, assuming that you already did the first two steps, do it like this:

https://www.jotform.com/62274708092155?session=client1 

https://www.jotform.com/62274708092155?session=client2

https://www.jotform.com/62274708092155?session=client3

4. Sign the eSignature field.

5. Send the session URL to your clients (individually).

Here's a test form showing the method: https://form.jotform.com/62275089189973?session=welvinjotform. 

b. Edit URL method.

1. Make all fields in your form as not required so that you can submit the form.

2. Add an autoresponder email (https://www.jotform.com/help/26-Creating-a-Form-Autoresponder). Make sure to select the client's email field as the recipient.

3. In the autoresponder email, add the edit link (https://www.jotform.com/help/40-How-to-Let-Users-Update-Their-Form-Submissions-at-a-Later-Date). 

4. Fill the form, sign your part and don't forget to put the client's email in the client's email field.

5. Submit the form.

6. Your client will receive the autoresponder email and will have to click the edit link to sign their part and fill the form with their information then submit it.

7. You will receive an additional email as soon the client re-submits the form.

I hope that helps. Let us know if you are confused with the steps.