Field update on screen load with incremental number "Transaction ID"

[PCgizmo]

I have a PA form there is a field "Transaction ID". Currently this field is manual input once the user clicks the submit button it populates an SP list  Inventory,"Transaction_ID" along with a number of other fields. This transaction_id will be the same for around 30 rows as the user inputs the inventory using the APP. 

What I have been failing to figure out Is how to have power apps populate the "Transaction_id" Field on the form with a +1 increment of the last transaction ID entered in the list increment starting at 100 . I would prefer the to happen on screen initialization. The ID will only exist in the field until the user clicks submit then is updated to the SP list.  The user may need to change the auto generated "Transaction_id" to a previous ID at some time.

Any foot hold to jump from or advice would be appreciate I have found forums and content that appears similar but did not really apply well.

[eka24 · Accepted solution]

Follow this video from Shane Young:
https://youtu.be/plKw_xfVfwY

Auto increment ID

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[PCgizmo]

Thank,

I'm not sure how i missed that one i have watch quite a few of his videos. This works up to the point of populating the field. I can see in the expression that it is pulling the correct info but it will not fill it into the field.