Rewrite formdata or change inlinecode

[acsgc]

Hello,

I'm not familiar with scripting at all, so I aprreciate any help I can get.
What I'm trying to do.

I have an router with a webform (https) and I need to fill out on field with data. When I sent the form this field is checking my input via inline code and is not accepting my input. These checks are nonsense and I need to get rid of these check or at least I need to set another value inside an if-Loop, where the script checks my value if it is between to given values. So I need to extend this.
Or alternativly I need to change my send data after the script checked my input and before sending the data to the router.
But how can I do that. Is there anybody who did something already?
Any help is welcome

Regards
Oliver

This is the code of the Config-Webpage where I need to change the highlighted value:

if(document.forms[0].ZDAT3_mtu.value=="" || isNValidInt(document.forms[0].ZDAT3_mtu.value) || document.forms[0].ZDAT3_mtu.value<1440 || document.forms[0].ZDAT3_mtu.value>1492)
{
alert(mld10_NetzwerkWAN);
document.forms[0].ZDAT3_mtu.value = "";
document.forms[0].ZDAT3_mtu.focus();
return false;
}

break;
}