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UA has a program called Premier Status Accelerator.
Summary -- for those with a purchased itinerary, you will be given the opportunity to purchase RDMs and then optionally, a similar number of PQMs PQPs. You are given two quantities to purchase (in the past it was X and 2X, but it has become more variable recently).. The numbers seem to have some relationship to the length of the existing itin.
The offer, for a purchased flight, will be found in your online reservation, It may say " Grow your award miles balance by purchasing Award Accelerator®. "
You will have to select the Award Accelerator in all cases, if you want Premier Accelerator also, then select it next.
The purchased miles post quickly -- generally less than an hour or two.
Purchase price does not count for PQDs PQPs
The offer will be available once ticketed and then until the itin is completely flown. The mileage offered decreases as segments are flown.
The cpm rate can change at any time -- up or down. See the thread on Rates.

Don't have an open itinerary? Just "purchase" dummy itin as refundable or non-refundable. After purchasing the RDMs/PQMs (must select the RDM offer to see the PQM PQP offer and must purchase the RDMs to purchase PQMs PQPs), cancel the dummy itin (if non-refundable do within the fee-free 24-hour cancellation window). The purchased RDMs/PQMs PQPs are non-cancelable / non-refundable and will stay purchased even if the dummy itin is canceled.
As before there will be two offers, if the offer is too many miles try a shorter itin, even a OneWay -- say SFO-LAX. There does seem to be a min offer level but it varies for the individual. You don't need to purchase to see the options, they will be displayed on the "United Travel Options" page, the page before you enter your credit card info,

Attempt to change the offer amount
-- try an international itin (seems to increase the offer)
-- try an international with a partner (seems to increase the offer)
-- try an OW itin (seems to decrease the offer)
-- try a short sub-500 segment (seems to decrease the offer)
-- try itins with a higher or lower price (offers seems to track relative price)
In practice, there is no close form method to get a specific amount. UA has your information.


Related thread -
United Airlines Award/Premier Accelerator Rates [2022]
United Airlines Award/Premier Accelerator Rates [2021]

United Airlines Award/Premier Accelerator Rates [2020]
United Airlines Award/Premier Accelerator Rates [2019]
United Airlines Award/Premier Accelerator Rates [2018]
United Airlines Award/Premier Accelerator Rates [2017]
United Airlines Award/Premier Accelerator Rates [2016]
United Airlines Award/Premier Accelerator Rates [2015]
United Airlines Award/Premier Accelerator Rates [2014]