how to add your own script to a sharepoint MasterPage

first required thing is SharePoint Designer.

after that open the website where you want to include your script, browse out to your specifique page and open it.

but an important thing before is to extract the page, like that if you fall out in an fatal error you can cancel the extract and you return to the begin state of the page.

after all this precaution you open the page, add your script :

and like if you put the script in body tags, it will fired on page load so the field test1 will take the string "Hello World" after load end.

thats a simple example, you can do more complected thing with that.

i would thank Robert Jones for helping me to make that work.