Open the Console by clicking f12 and selecting the console tab, and then try to write:

document.dlna_form.samba_active.value = "Yes";

check if it even works by writing:

document.dlna_form.samba_active.value;

if not, then try to find it's ID and then write:

document.getElementById('the_ID').value = "Yes";

when your done just write:

document.dlna_form.samba_active.value;

to see if it has changed.

FYI: if the server has some other validation on the server side then what your doing will not work, you can try some other methods by sending requests of GET or POST with different input data based on the server that your trying to change.