Custom field updating with a random value while using that field through connected app

I have a field securoity token under contact object and we are trying to generate a token using external application with the connected app and we are not able to update the field as there is a default random value generated on that field.
 

Custom field updating with a random value while using that field through connected app

I have a field securoity token under contact object and we are trying to generate a token using external application with the connected app and we are not able to update the field as there is a default random value generated on that field.