Add an Action button item type to the template and go to the Button 1 tab.  Tick “web link” and, in the “lookup field”, specify the unique name of the item that holds the URL to open.  This means that the URL can be changed with a minimum amount of fuss.

The URL may be part of the job information that is sent down from the server, in which case the device user will not need to modify.  However, if it is, or subsequently becomes, part of the information that is to be collected the field can become available to them to fill in themselves if the following method is used.

Create a Text item on the form and then put in a default value of the required URL - it may be possible to use

You could also hide the Text item by putting on a dummy display condition like resultValue;xxx;equals;999, thereby preventing the device user from being able to amend it.